Showing papers in "Biological Reviews in 1946"

The physiological significance of the phosphomonoesterases

Abstract: Summary Researches of the past ten years bearing on the physiological significance of the common acid and alkaline phosphomonoesterases may be summarized in the following facts and concepts, (i) The alkaline phos-phomonoesterase appears to be a salt-soluble, the acid a water-soluble, substance; both consist of a metallic coenzyme and a protein-like carrier. Neither depends oh sulphydryl groups for activity, but the alkaline phosphatase seems to require amine and phenolic hydroxy! groups; it is affected only by strong oxidizing agents. There is not yet any incontestable evidence that alkaline phosphomonoesterases of different sources are not identical. (2) Phosphatases probably catalyse only hydrolyses, not syntheses. (3) The content of both phosphatases in the blood serum is characteristic of age and physiological state, and is used in the diagnosis of bone and liver disorders; biliary obstruction results in elevation of serum alkaline phosphatase probably because phosphatase is normally excreted by way of liver and gut, but other explanations are possible. Diet influences serum alkaline phosphatase level. (4) The study of phosphatase function has been advanced by the development of techniques for the histochemical demonstration of acid and alkaline phosphatases. (5) It is certain that alkaline phosphatase is indispensable in normal ossification; it is generally instrumental in abnormal calcifications, but in a few cases calcification occurs without it. (6) Glycolysis seems to accompany normal bone formation, but this fact does not clarify all the unsolved problems of ossification. (7) Hormones and vitamins influence both alkaline phosphatase activity and bone formation. (8) In kidney tubules alkaline phosphatase probably functions in dephosphorylating sugar molecules that are removed from the tubular fluid by phosphorylation. The enzyme is present at other sites where it might serve in a similar manner. (9) Phosphatases are commonly found in the cytoplasm of growing, regenerating, and secreting cells in which protein synthesis is being carried on; there appears to be a correlation in such cells between content of pentosenucleic acid and of phosphatase. In dividing nuclei, phosphatase parallels desoxyribonucleic acid. (10) Acid phosphatase is found in seminal fluid, and may help to nourish the sperm. (11) The occurrence of two common phosphomonoesterases with widely different pH optima may indicate a mechanism for allowing two dephosphorylating steps to go on simultaneously in a single cell. In tissues which carry on much traffic in glycogen, acid phosphatase is possibly provided to prevent bone formation. (12) The actual role which the phosphatases play in the physiology of the organism will be understood only with elucidation of the complete enzyme systems of which the phosphatases are a part.

Bacterial viruses or bacteriophages.

Abstract: Summary (i) The following factors are held to be chiefly responsible for recent progress in the field of bacterial viruses: (a) the introduction of ‘one-step growth experiments’, (b) the concentration of attention on a small group of bacterial viruses, (c) the proof that virus-resistant mutants originate independently of the presence of virus, (d) the application of the electron microscope. (2) Types of virus morphology, as revealed by the electron microscope, are described. The virus particles of any one strain are of uniform size and shape. Many strains show a ‘head-and-tail’ structure. Sizes between 45 and 100 mμ have been found. (3) Classification of a group of seven viruses by serological cross reactions agrees perfectly with classification by morphology. (4) A method for determining the origin of mutations of bacteria from virus sensitivity to virus resistance is outlined. There is evidence that the classification of viruses by cross-resistance tests is of little value in establishing natural relationships between viruses. (5) The phenomenon of ‘lysogenesis’ is discussed, and certain ambiguities in its definition are pointed out. (6) The physiological basis of resistance of a bacterial strain to the attack by a specific virus is discussed. Such resistance is generally coupled with failure on the part of the bacteria to fix the virus in question by adsorption. Evidence shedding a new light on the process of adsorption is reported. (7) Bacterial viruses can mutate in such a way that their host range is altered. These mutations occur during the multiplication of the virus in its host. (8) The technique and terminology of ‘one-step growth experiments’ are briefly described. (9) When a bacterium is attacked simultaneously by several virus particles of the same strain or of different strains the bacterium reacts as if it had been attacked by only one particle (mutual exclusion effect). A detailed study of this phenomenon has led to the assumption that the membrane of the bacterium becomes impermeable to other virus particles after one particle has entered the cell (penetration hypothesis). The excluded virus particles may affect the yield of virus of the successful type (depressor effect). (10) The adaptation of the technique of the ‘one-step growth experiment’ to biochemical studies of virus multiplication is described. (n) The controversies regarding the nature of bacterial viruses are briefly touched upon, particularly with reference to the evolutionary standing of these viruses. (12) It is pointed out that a thorough study of bacterial viruses may lead to a better understanding of all types of viruses.

The conducting system of the vertebrate heart.

Abstract: Since the middle of the seventeenth century, controversy has raged about the question whether the rhythmic contraction of the heart is due to the action of nerve elements (extrinsic or intrinsic), or is a specific immanent property of the cardiac muscle itself. The former, neurogenic theory, was in predominant favour until the work of Engelmann (1875 et seq.) and Gaskell (1882 et seq.) placed the opposing, myogenic hypothesis, on a firmer foundation. Of the host of workers who have subsequently studied this problem, the majority favour the myogenic initiation and conduction of the impulse for cardiac contraction. Limitation of space forbids any detailed discussion of the voluminous literature. concerned with this controversy, and the present review is therefore mainly devoted to the histological and physiological properties of the various types of muscle in the vertebrate heart. Even in this restricted field, the literature is so extensive.that only those work which are considered to rank foremost in the advances in knowledge in this subject receive attention. Although the idea of a regular sequence of ,events in the cardiac cycle was appreciated by earlier workers, the classical experiments of Stannius (1852), comprising the separation of the various chambers of the frog’s heart by ligatures applied at the sinuatrial (s.-A.) and atrio-ventricular (A.-v.) junctions, were pre-eminent as a basis for the subsequent more careful and detailed studies concerning the site of origin and the mufee of the wave of cardiac contraction. Stannius’s results were at that time interpreted in the light of the prevailing neurogenic theory, and were thought to indicate that the contractions of the several segments of the heart were controlled by the ganglia at the SPA. junction

The loss and restoration of regenerative capacity in the limbs of tailless Amphibia.

Abstract: Summary (I) When the hindlimbs of Rana temporaria are amputated in the proximal part of the shank during stage I of metamorphosis they regenerate, but they lose their regenerative capacity at stage II. The regenerative capacity is retained longer in the distal than in the proximal parts of the limbs, which is due to the process of histological differentiation taking place in a proximal-distal direction. (a) When the amputation is made during stage I the mesodermal tissue of the limbs dedifferentiates, regenerative blastemata are formed and the limbs regenerate. When the amputation is made during a later stage the mesodermal tissue does not dedifferentiate, blastemata are not formed and the limbs do not regenerate. (3) By a traumatization or a transplantation it is possible to excite complete regeneration at those stages in which the opposite (control) limbs of the same animals do not regenerate. (4) The restoration of the regenerative capacity of the limbs of tadpoles is brought about by the dedifferentiation of the mesodermal tissue in the distal part of the stump and not by an acceleration of the process of cell proliferation. (5) The regenerative capacity of tadpoles' limbs may be extended by means of repeated regeneration to a late stage of metamorphosis, or. indeed, up to the completion of this process. The re-newal of the regenerative capacity is dependent on the retention of the mesodermal tissue of the limbs at a level of histological differentiation which permits of de-differentiation. (6) The regeneration of the limbs of Anura and Urodela occurs in two phases: during the first phase the blastemata are formed almost entirely without mitosis, by the direct liberation of cells from the dedifferentiating mesodermal tissue; during the second phase the blastemata are formed mainly through intensified cell proliferation. (7) Tadpoles' limbs regenerate much more quickly, their mesodermal tissue dedifferentiates much more profoundly, and the mitotic coefficient is much lower than in the case of Urodela (axolotls). The difficulty of dedifferentiation, therefore must have a stronger inhibiting influence on the regeneration of tadpoles' limbs than on those of axolotls. (8) During the process of metamorphosis the internal humoral medium of the animal undergoes changes due to which there is an acceleration of the histological differentiation of the limbs and a suppression of the possibility of dedifferentiation and the formation of blastemata. (9) During metamorphosis, together with the loss of regenerative capacity, changes take place in the properties of the epithelium and of the mesodermal tissue of tadpoles' limbs. Changes in the mesoderm have a stronger and more profound influence on the regenerative capacity than changes in the epithelium. (10) Changes in the nervous system have no important effect on the regenerative capacity of tadpoles' limbs, (n) The loss of regenerative capacity in tadpoles' limbs is connected primarily with histological differentiation. This may to a certain extent occur independently of the changes that take place in the internal humoral medium, but in a typical case it is accelerated by these changes. (12) The main factor causing a loss of regenerative capacity in tadpoles' limbs is the histological differentiation of the mesodermal tissue and its inability to dedifferentiate. (13) To restore the regenerative capacity of the limbs of Anura the formation of blastemata must be effected, for which it is primarily necessary to cause dedifferentiation in the mesodermal tissue of the stump.

The concept of nerve-muscle specificity.

Abstract: Summary (1) The concept of nerve-muscle specificity postulates the existence of an inherent, specific attraction between given striated muscle fibres and given nerve fibres or neurons. (2) The chief theories of nerve-muscle specificity, those of Furbringer and Cunningham, are considered. Furbringer's theory–that there is an unvarying, specific relationship between particular macroscopic, peripheral nerve branches and particular muscles–is founded upon a false embryological concept and is completely negated by both comparative and experimental data. Cunningham's theory–that there is a constant relationship between given neurons and given muscles but which recognizes variability in pathways adopted by axons and hence in macroscopic innervation –has some comparative anatomical support but is denied by the findings of experimental embryology. The theory that muscular nerve supply is inconstant, and that there may be secondary innervation, especially in phylogeny, is more in accordance with known facts. (3) Existing knowledge of the development of nerve-muscle relationships–the outgrowth of nerve fibres, the formation of peripheral nerve patterns, and the establishment of terminal connexions–is reviewed. Although a number of important questions remain to be answered, there is nothing to indicate the existence of anything approaching a true nerve-muscle specificity. (4) It is concluded that present evidence is against the concept of nerve-muscle specificity. Thus resemblances in muscular nerve supply must merely reflect general similarity in development. Because of this, innervation is of great aid in homo-logizing the muscles of animals belonging to one vertebrate class, but not in homologizing the muscles of animals of different classes. It is shown that urodeles so differ from other tetrapod vertebrates respecting nerve-muscle relationships that they form a group unto themselves.

The theoretical aspect of insect metamorphosis.

Abstract: Summary .(i) The general theory is developed that metamorphosis is not the activation of imaginal rudiments but a repetition of the developmental processes occurring during embryogenesis. Furthermore, the developmental phenomena accompanying ordinary ecdysis are comparable to metamorphosis and hence also to embryogenesis. Insect life histories consist therefore of a series of repeated developmental cycles all similar in essence to embryogenesis. In insects without metamorphosis these cycles are all alike and no abrupt change takes place. In metaboious forms the earlier cycles are modified to varying degrees. In the Hemimetabola the earlier cycles are subject to the influence of inhibitory hormones and the adult features appear only at the last ecdysis. In the Holometabola the larval cycles are subject to a much more powerful ‘suppression’ influence which checks development and is maintained almost at full level throughout larval life. At metamorphosis this influence is removed and a full development can then take place. Hence degree of metamorphosis depends on the degree of influence of the suppression factor. (2) In the embryonic development of Calliphora the mid-gut develops in three sections: (a) the pro-enteron, formed by direct transformation of the inner end of the ‘stomodaeum’ and lying between the stomodaeal membrane and the anterior imaginal ring, (b) the mes-enteron, formed from polar rudiments (or in other types from the proliferations from the inner end of the stomodaeum and proctodaeum), and (f) the met-enteron, formed by direct transformation of the inner end of the ‘proctodaeum’, carrying the Malpighian tubules as anterior appendages, and situated between the proctodaeal membrane and the posterior imaginal ring. It is believed that many widely divergent types, including Aptera and Orthoptera, as well as Holometabola, conform to this mode of development. In Calliphora, at metamorphosis, the anterior imaginal ring forms a new ‘pupal stomodaeum’ which produces an adult pro-enteron in the same way as the corresponding larval structure arose in the embryo. Similarly, the posterior imaginal ring recreates the pupal proctodaeum and eventually forms an adult met-enteron. In spite of serious gaps in our knowledge it is believed that most of the Holometabola conduct their metamorphoses in this way. It is also probable that in many cases new mid-gut cells are budded off from the imaginal ring to form a new adult mes-enteron. (3) The basic similarity between the developmental cycles associated with metamorphosis and those associated with ordinary ecdysis is shown by the experimental results derived from Rhodnius. Here any ecdysis can produce nymphal or imaginal characteristics according to the presence or absence of inhibitor hormone. ICpithelial renovation in the mid-gut, such as occurs at metamorphosis, takes place at all ecdyses, except in the higher Diptera and Hymenoptera. The formation of the imaginal Malpighian tubules at metamorphosis in the Hymenoptera is similar to the mode of origin of new tubules in ordinary instars in Blatta. (4) The suppression of development in early instars is shown in several ways. Cell hypertrophy, characteristic of many larvae, is due to a high degree of polyploidy. Thus cell division, but not chromosome division, is suppressed during larval stages. At metamorphosis, when the suppression influence is removed, rapid mitosis occurs, and the correct chromosome numbers are restored. Epithelial renovation in the mid-gut usually occurs at each ecdysis (as well as at metamorphosis), but in the higher Diptera and Hymenoptera it is suppressed during larval stages. Imaginal cells are not imaginal rudiments but merely suppressed renovation cells, which resume their normal activity at metamorphosis because the suppression influence is then removed. If a new adult mid-gut is produced it probably always arises from the imaginal rings as in Calandra. The imaginal Malpighian tubules of the higher Hymenoptera are homologous with the secondary tubules which arise in every instar in Blatta. Their non-appearance prior to metamorphosis is thus an indication of suppression of their development during larval stages. In the vast majority of Coleoptera, Lepidoptera, and Diptera secondary tubules are permanently suppressed and the larval tubules become the adult organs. The lifting of the influence of the suppression factor at metamorphosis shows that it has much in common with the inhibitory hormone in Rhodnius. However, the latter has not been shown actually to suppress development, although at present only its action on the hypodermis is at all well known. (5) Variations in larval forms can be explained by assuming that they are due to varying degrees of intensity of action of the suppression influence. They may be arranged in a series such as nymph, campodeiform larva, various degrees of reduction of the campodeiform type, caterpillars, grubs and maggots. Such a series then represents successive reduction due to progressively more severe suppression in embryonic and larval stages. Hypermetamorphosis is the result of varying degrees of suppression at the various ecdyses of an individual life history. The protopod and polypod instars of certain of the parasitic Hymenoptera are precociously hatched embryos and not definitive larval forms like the rest.

The experimental basis for hybrid maize.

Abstract: Summary The development of superior maize hybrids and their rapid acceptance by farmers constitutes one of the most important advances in American agriculture within the past century. In 1944 hybrid maize plantings occupied approximately 83 % of the maize acreage in the Corn Belt. The increase in yield resulting from the use of hybrid maize was estimated at 669,480,000 bushels in 1943. The production of hybrid seed involves four separate steps: (1) development of inbred lines and testing of these in hybrid combinations, (2) multiplication of seed of the component lines, (3) seed production of the foundation single crosses, and (4) production of double-crossed seed used for commercial planting. The earlier methods used in maize improvement included mass selection, varietal hybridization and ear-to-row selections. These methods were of limited value and are now of interest largely from a historical standpoint. Inbred lines are developed by self-fertilization accompanied by selection. Selection is practised for the various characters which are expressions of vigour. Positive and significant correlations have been reported between yields of the hybrids and yield, plant height, ear length, ear diameter, shelling percentage, number of ears per plant, leaf area, brace roots and root volume of the inbred parents. The final evaluation of an inbred line is determined by its performance in hybrid combinations. The top-cross or inbred-variety cross is commonly used for the preliminary evaluation. Subsequently the more promising lines are tested in single-cross combinations. Evidence has been presented indicating that combining ability of inbred lines is heritable. In new lines isolated from single crosses, a greater proportion of high-yielding combinations are obtained when the component lines are derived from unrelated parentage. Single-cross performance data may be used to predict the performance of double-crossed combinations. The most efficient method of estimation is based on the average performance of the four non-parental single-cross combinations. The order of pairing of the four component lines of a double cross may have a marked effect on variability and yield. In general the highest yielding double crosses are those that combine single crosses differing most widely in parentage. In the direct inbreeding of open-pollinated varieties the great majority of lines are discarded after preliminary testing. The remaining lines produce desirable hybrids, and these are used as source material for a new cycle of inbreeding and testing. These cyclical repetitions of inbreeding and testing are designated as cumulative selection. Second generation seed is sometimes used as parent stock in crossing fields. If genetic identity has been maintained double-crossed seed produced from F2 or F3 advanced generation single-cross stocks will produce yields which are essentially similar to the same double crosses produced from F1 single crosses. The use of F2seed for commercial planting invariably results in rather large decreases in yield. Synthetic varieties are advanced generations of the open-pollinated seed mixtures of a number of inbred lines or of hybrids among them. When the component lines are selected on the basis of combining ability yields of the resulting synthetics approximate yields of standard double-crossed combinations. Several new methods of isolating and testing inbred lines are under investigation. All of these involve some aspect of cumulative selection or early testing. Early testing is based on two assumptions: first, that there are marked differences in combining ability among the plants of a population selected for inbreeding, and secondly, that a selected sample based on tests of combining ability of S0 plants provides a better sample for further inbreeding and selection than a more nearly random sample from the same population selected on visual appearance alone. Gametic selection, one aspect of early testing, is based on the assumption that if zygotes of a given combining ability occur with a frequency of 1 % then gametes having a corresponding combining ability should occur with a frequency of 10 %. The problem of identifying these superior gametes is discussed. Recurrent selection for specific combining ability involves selection for combining ability with a minimum of inbreeding. No experimental data are available to evaluate the method and possible advantages and limitations are presented. The general problem of heterosis is one of the most important problems in genetics at the present time. Various aspects of this problem are discussed. Recessive characters probably have little or no influence on heterosis. Data on growth rates have thrown some light on the amount of stimulation resulting from hybridization but have been of limited value in providing a physiological explanation of the nature of the stimulus involved. Convergent improvement studies indicate that the dominant genes from two lines may be concentrated in a single line by recurrent back pollination, accompanied by selection. The data available indicate that dominant favourable genes are more important in heterosis than physiological stimulation resulting from genetic diversity. Attempts to assess the relative importance of general and specific combining ability have little critical bearing on the heterosis problem at present. Inversions have been suggested as a possible tool for determining the chromosomal distribution of factors responsible for heterotic effects. Data are limited but suggest the method is feasible. Progress in the development of disease-resistant lines and hybrids is limited by the lack of suitable techniques for inducing epidemics at will. Information on resistance is reviewed briefly. Relatively little work has been done in developing inbred strains resistant to insect pests except in the case of the European cornborer. The results obtained indicate ample genetic variation for progress when an adequate infestation can be obtained. Maize varieties are extremely variable, and striking responses to selection have been reported. The Illinois selection experiments for high and low oil and high and low protein are the most notable. After 29 years of selection for high and low values respectively oil percentage had been changed from 4.68 to 9.86 and 1.51. The corresponding modification in protein percentage was from 10.93 to 16.60 and 4.68. Further results and opportunities in breeding for chemical composition are mentioned.

The natural history of whalebone whales

Abstract: Summary (1) The whaling industry has provided both the facilities and the stimulus for modern research on the general biology of whales. The principal methods of investigation are (a) anatomical examination, (b) observations at sea, (c) the marking of whales, (d) analysis of the statistics of the whaling industry, (a) The whalebone whales are migratory animals, inhabiting high latitudes in summer where food is plentiful, and moving into warmer waters in winter where there is little or no food, but where breeding takes place. The Greenland right whale does not move far from the Arctic regions and is not found in the southern hemisphere. The black right whales of the north and south do not migrate far and are separated by a wide tropical belt. The humpback migrates from the polar ice to the equator, and frequents tropical coastal waters in the winter months. In the southern hemisphere it is segregated into several communities which have separate migration routes, and between which there can be little interchange. Blue and fin whales undertake less regular and extensive migrations. They are not segregated like the humpbacks, but show a slight tendency to concentrate in the same regions. Grey whales inhabit the North Pacific and undertake regular migrations along the coasts of North America and in Japanese waters. There is less information on the distribution of the sei, lesser rorqual, pigmy right, and Bryde's whale. (3).Certain planktonic Crustacea form the principal food of the whalebone whales. In the Antarctic they feed virtually exclusively on the shoals of Euphausia superba. In the northern seas the diet seems to be more varied. Meganyctiphanes norvegica is probably the most important food organism in the North Atlantic, but further investigations are needed. Little food is taken in winter, though fish and small quantities of other Crustacea are sometimes eaten. (4) Examination of the reproductive organs and measurements of foetuses at different times of year show that breeding mainly takes place in winter and that the period of gestation is about a year. Normally one young is born at a time, and the usual interval between successive pregnancies is probably two years. This applies to blue and fin whales, but other species are probably similar. Blue and fin whales are believed to become sexually mature in about two or three years. The old corpora lutea of the ovaries persist and accumulate, and constitute the best indication so far found of the age of an adult whale. There is some evidence that the rate of increment is about one per year, but this again needs confirmation. The largest recorded number is fifty-four. Indications of periodic growth in the baleen plates constitute a new method of determining the ages of young whales. The rate of growth is faster in the anterior than in the posterior part of the body. (5) Whalebone whales are more plentiful in the southern than in the northern hemisphere. The sexes are nearly equal. The existing ratio of blue, fin and humpback whales is estimated to be of the order of 15, 75 and 10 respectively in the Southern Ocean, but no estimate has yet been made of the absolute numbers in the populations. Most whaling is carried out by the Antarctic pelagic factories, and little is done now in the northern hemisphere. The stocks of blue and humpback whales have been depleted by the modern industry, but fin whales have been less affected, and progress has been made in the international regulation of whaling. (6) In the future it will be necessary to continue research to some extent on the same lines as before, but new or modified methods could be developed, and aircraft and modern technical devices might be used with advantage.

Interrelationships and taxonomy of the digenetic trematodes.

Abstract: Summary The purpose and objective of classification is to portray genetic relations by taxonomic symbols. Adoption by trematodes of a parasitic mode of life has induced changes in nutrition, physiology, morphology, life cycles and development. These adaptive modifications have resulted in both convergent and divergent evolution. Criteria for determining genetic relations must be assessed with caution and no single measure can be accepted as reliable or adequate. Although certain features appear to be correlated, they may have developed independently and the evaluation and interpretation of diagnostic characters is very difficult. Types of adhesive organs and the extent of their development may be associated with the site of infection, but life histories and the type of development afford more significant information concerning genetic relations. Consideration of morphological and developmental data lead to the conviction that in the Digenea, the higher taxonomic units have little phylo-genetic or systematic significance. It is proposed therefore, to suppress the orders Gasterostomata and Proso-stomata, and in the latter the suborders, Amphistomata, Distomata, Monostomata, Strigeata, Bucephalata, Schisto-somata and Clinostomata. Although superfamily groups may be recognized, their limits and interrelations are not yet clearly defined and their erection should await more complete information. Former systems for classification of adults were based on morphological features, some of which have proved unreliable, and life-history data are yet too meagre for final evaluation and application. Classifications of larval stages, based on types of asexual generations or on larval structures such as form of tails, presence of stylets, etc., have proved more unsound than classifications of adults. Only an analysis, evaluation and correlation of all data can produce a truly natural system. Such a system must be based on and portray the evolutionary history of the trematodes. Phylogenetic considerations are reviewed and applied to taxonomic problems.

The bionomics of coprophilic protozoa

Abstract: Summary The coprophilic Protozoa are forms capable of living in faeces. They form a loose ecological group of considerable practical and economic importance. The development of the conception of coprophilic Protozoa is traced in the article, and it is shown that at first they were frequently confused with or taken for parasitic species, many of them being considered as pathogenic. Further research showed that they were in reality free-living species which had become adapted to life in dung. The faecal environment is peculiar in being characterized by abundance of decaying organic matter, vast numbers of bacteria with eonsequent restriction of free oxygen, and restriction of fluid to small pockets and channels between the solid particles. Hence the only Protozoa able to live in undiluted faeces are polysaprobic species whose methods of locomotion permit them to move in restricted volumes of fluid or in a viscous medium. Some coprophilic Protozoa are holozoic, while others are saprozoic. Most of them flourish best at low temperatures, but they are tolerant of wide variation in the hydrogen-ion concentration of the medium and can withstand temporary deficiency of oxygen and abnormally high concentrations of carbon dioxide. They are all well adapted to withstand desiccation by anabiosis or cyst formation. Although they can live and multiply in normal faeces so long as they remain sufficiently moist, many of them are not able to survive in pathological faeces from cases of diarrhoea and dysentery. The factors inhibiting growth in this medium are the presence of drugs, or of unaltered bile salts, an unusually high osmotic pressure, or the admixture of urine with the fluid stool. The presence of mucus in the faeces, on the other hand, is favourable to the growth of coprophilic Protozoa. Although urine is lethal to the active forms, it probably does not act as an important limiting factor in nature, since with normal faeces it merely bathes the surface and does not become admixed to any extent. A classified list is given of all the Protozoa reported from human and animal faeces. While the drying up of the faeces leads to the disappearance of active Protozoa, the dilution of the faeces artificially or by rain leads to the development of a succession of forms analogous to that which takes place in a hay infusion or in the waters of a polluted river during self-purification. Coprophilic Protozoa may be classified, according to their degree of adaptation, into three groups, namely: (a) Oligocoprophilic forms, able to live in faeces only after dilution; (b) Mesocoprophilic forms, able to live in some undiluted faeces, especially if of loose consistency, but flourishing only after dilution; (c) Polycoprophilic forms, able to flourish in undiluted normal faeces. The medical and veterinary importance of the coprophilic Protozoa lies in the possibility of confusing them with or taking them for true parasitic forms. Some of the classical cases of pseudo-parasitism, due to this cause, are described. Coprophilic Protozoa gain access to the faeces either by passing through the alimentary canal in the encysted condition, or by contamination of the stool after defaecation by air-, water-, or soil-borne cysts. The former are referred to as endogenous forms, the latter as exogenous. Coprophilic Protozoa may be distinguished from parasitic species by the fact that they may be readily cultivated at room temperature but die out at 37o C. A close ecological relationship exists between the Protozoa of sewage and the coprophilic Protozoa. The latter occur characteristically in sedimentation tanks and Imhoff tanks, but are also represented in sprinkling filters and activated sludge. They play a part in the. purification of sewage. A similar close ecological relationship exists between coprophilic and soil Protozoa, almost all the coprophilic species occurring also in the soil from which they were originally derived and to which they are continually being returned. Coprophilic Protozoa also occur in natural manure, and it has been suggested that the loss of nitrogen from stored dung owing to the activities of bacteria and moulds might be averted by ‘seeding’ with suitable coprophilic forms. Many species of coprophilic Protozoa are useful indicator species, and their presence in drinking-water supplies is suggestive of contamination with faecal matter or sewage. Coprophilic Protozoa are of evolutionary significance, since they recapitulate the various steps by which free-living species may have become adapted to the parasitic habit, namely: (a) Close relationship to established parasitic forms. (b) Occasional excystment and temporary existence in the lower bowel, (c) Facultative parasitism in cold-blooded hosts, (d) Established parasitism, but with the ability to lead a coprozoic existence temporarily. The factors involved in this change are discussed and the conclusion reached that coprophilic forms may evolve into true parasites.

Antigens, antibodies and genes.

Abstract: Summary This article is a survey of a few pertinent findings about antigens as they occur in one species of bacteria, the pneumococcus, and in the cells and organs of mammals and birds. The main points of general biological interest emphasized are as follows: (1) The fact that carbohydrates, not proteins, are responsible for the specificity of the different types of pneumococci points the way to a comparable concept of specificity in organisms other than bacteria. That is, substances other than proteins, if linked to proteins, may account for some part of known specificities. (2) Changes within each type of pneumococcus may take place from a smooth, capsulated form to a rough, non-capsulated form, and vice versa. Under certain conditions a rough form of one type may be changed to the smooth form of an entirely different type. The activating principle in one such change has been found to be a nucleic acid. (3) The antigens of the blood cells of man are gene-determined. There are, however, antigens of the blood cells, tissues or organs of other species which have not yet been explained genetically. (4) It has been possible by immunological techniques to make a sharp distinction between the cells or sera of closely related species. Reagents prepared by absorbing the antisera to the cells or serum of one species by the corresponding antigens of the other were reactive with the homologous antigen, but not with that used in the absorption. From these results it may be deduced that each species possesses ‘species-specific’ antigens in comparison with a related species, as well as those common to the two species. (5) In bird species which produce viable hybrids the antigens have been found to be gene-controlled. (6) The cellular antigens are generally thought to be the more or less direct products of their causative genes. However, a few instances are known of cellular antigens which are the result of complementary action of genes. Also, within the group of genes which act directly on the respective antigens there appear to be some which singly are capable of effecting the same combination of antigens as two (or more) alleles may produce.

The systematic anatomy of the vegetative organs of the angiosperms

Abstract: Summary Historically the study of the anatomical structure of the vegetative organs of the flowering plants for the purpose of classification has developed slowly owing to the rather wide gulf which at one time existed between taxonomy and other branches of botany. In recent years the subject has been given a new stimulus through a wider interest in wood structure, chiefly amongst foresters and timber technologists. Further stimulation has been caused by the formation in 1930 of an International Association of Wood Anatomists. Satisfactory investigations in systematic anatomy must be based on accurately named material which is not always easy to obtain, especially from tropical regions. Reference collections of microscopical slides are as necessary to the systematic anatomist as are herbarium specimens to orthodox taxonomists. Representative slides made during important systematic investigations should be deposited at a recognized taxonomic centre so that they can be consulted by future workers. Arrangements to facilitate the exchange of slides and information between institutions where systematic anatomy is studied are important. There is at present no purely scientific method of identifying or classifying botanical material by microscopical methods. Success is achieved, in practice, largely by memorizing anatomical features, by experience and intuition. Circumstantial as well as strictly botanical information is often helpful. Dichotomous and similar keys are useful, but of limited value when dealing with a large group of plants such as the whole of the dicotyledons. Reference is made to a multiple-entry perforated card index system now in common use for the identification of timbers, and it is suggested that semi-mechanical methods of this kind could usefully be applied to other branches of systematic anatomy or even to taxonomic studies based on exomorphic features. It is most important that characters selected for diagnostic purposes should be those which practical experience shows to be reasonably constant and not liable to modification by environmental changes. The practical value of systematic anatomy cannot be fully assessed until much more descriptive work has been done. It is known that the method is of definite value in the interpretation of genera, and that it often affords evidence concerning the interrelationships of families. It is of less value in the interpretation of species and taxonomic units of still lower rank. Systematic anatomy is of economic importance, since it enables timbers, fibres, crude drugs and similar botanical material to be identified even when floral characters are lacking. The approximate affinities of sterile herbarium specimens may also be established by this method. A more complete knowledge of the systematic anatomy of present-day plants would be of assistance to palaeobotanists. Recent studies have shown how graded series of anatomical features in the secondary xylem of the dicotyledons and in the metaxylem of the vascular bundles of the monocotyledons can be used to indicate probable phylogenetic sequences. There are difficulties in comparing woody plants and herbs from this point of view owing to the small amount of secondary xylem in the latter. This fact sets a limit to the value of phylogenetic studies which are based on wood structure alone. The basis of taxonomy is being broadened at the present time, and there seem to be reasonable prospects that, in the future, the facts of systematic anatomy, together with those of cytology, embryology, genetics and other branches of botany, will play an important part by helping to give a deeper significance to systems of'classification than they have done in the past.