摘要:

SummaryThe causal relationships responsible for the early organization and subsequent structural configuration of the vertebrate central nervous system are poorly understood. This is due in part to the complex nature of the subject but perhaps even more to the fact that causal neurogenesis has not yet attained full status as an independent biological discipline. Those general principles which are known derive largely from the separate fields of experimental embryology and descriptive neurology and are based, for the most part, on a random collection of non‐integrated facts. The source and nature of the data are probably adequate; what is needed is a conscious, co‐ordinated effort to envisage this field of study in its totality and, for immediate purposes, as an end in itself.1. The early formation of the medullary plate is dependent upon the inductive effect of the mesodermal substratum. Physical contact is probably necessary. Organization of neural ectoderm into medullary plate and tube in the absence of underlying tissue has been reported, but these results are probably due to the disintegrating action of the culture medium upon the inner, uncoated plate cells. The autolysis produced liberates neurogenetic substances responsible for the inductions obtained and the process is considered a pathological one. The morphogenetic forces responsible for the folding of a flat medullary plate into a hollow tube are probably intrinsic to the neural ectoderm itself. An increase in the contractile tension of the superficial gel layer of the external ends of the plate cells may be an important factor. Neither differential water absorption nor localized cellular proliferation seems to play a part in the process. The establishment of polarity and morphological gradients within the neural axis are relatively independent of substrate tissue. Throughout the early stages of neurogenesis the neural ectoderm plays a more active role than has previously been supposed.2. There exists no valid evidence that neuromeres represent an inherent metamerism of the early neural axis. The segmental arrangement of the spinal ganglia, and probably dorsal and ventral roots as well, is imposed from without by virtue of the proximity of the segmenting axial mesoderm. Mesodermal segmentation is primary; ectodermal segmentation is secondary. The organizing effect of the mesoderm upon the segmentation of the neural tube is more pronounced than certain aspects of the mesodermal‐ectodermal relationship obtaining in earlier stages.3. The theories of Kappers (neurobiotaxis) and of Bok (stimulogenous fibrillation) are inadequate for solving the complexities of neural differentiation. Neither theory is buttressed by critical experimental evidence and both are highly speculative in nature. Neurobiotaxis and stimulogenous fibrillation still remain thedues ex machinaof neurology.4. Nothing decisive is known concerning the important phenomena of cellular proliferation and differentiation within the central nervous system. Failure to delineate clearly between these two processes has caused some confusion in the past. Cell number is controlled to some extent by position of a part within the anterior‐posterior gradient; more anterior regions of the neural tube are less dependent upon such a position effect. The definitive number of differentiated cells constituting a specific region is regulated by the size of the peripheral field. This is true of intramedullary as well as extramedullary cells and is common to all major vertebrate groups studied. The mechanism whereby non‐nervous tissue effects this control of neuroblasts is not known. Experimental studies differ in their interpretation as to whether regulation occurs during the initial stage of differentiation or is implemented through the subsequent atrophy of already differentiated cells. Invasion of an undifferentiated field by nerve fibres from a neighbouring region has been thought to initiate the differentiation of the indifferent neuroblasts. Critical evidence for such a thesis is as yet lacking. There is good experimental reason to believe that the non‐nervous periphery affects differentiation alone, while proliferation of indifferent cells is influenced to a great extent by intracentral factors.5. The arrangements of nerve fibres into specific tracts and the polarity of their growth is generally supposed to be influenced by pre‐existing fibre pathways, particularly the fasciculus longitudinalis medialis. Regenerating intramedullary fibres have been observed to traverse preferential routes through the embryonic cord and to exhibit marked polarity in their growth. Substrate configuration and mechanical forces are probably of great importance in directing fibre growth. The reason for the formation of the standard commissures and fibre decussations is not known. The operation of purely mechanical forces within the central nervous system appears inadequate to explain all the intricacies of fibre pattern observed.6. The precise point of entrance of sensory components of cranial and spinal nerves into the central nervous system has been thought to be determined by localized attraction centres of high metabolic activity. Indirect evidence exists for such a supposition but critical proof is lacking. Ectopic sensory nerve roots usually enter the brain wall at definite loci and their dorso‐ventral (medio‐lateral) orientation is probably determined by the position of correlation tracts of corresponding function. The place of exit of motor roots can be experimentally altered, but nothing is known concerning the causal relationships involved in their normal egress.7. Structural regulability of the embryonic central nervous system reaches a high level in amphibians, less so in fishes and mammals. Specialized cell types are determined early in ontogeny but may be replaced from indifferent cells at relatively late stages. The cytogenesis of various cell types is probably influenced by factors extrinsic to the cells themselves. Anatomical and functional repair of intramedullary fibres is possible. Special inhibiting factors are probably responsible for failure of proper regeneration rather than any inherent difference between central and peripher

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1948.tb00455.x

出版商:Blackwell Publishing Ltd    

年代:1948

数据来源: WILEY

摘要:

SummaryThe Euglenineae form a well‐defined natural group comprising a great number of species, the subdivision of which into families is difficult to make.Among taxonomic features, the basic shape of the cell body is fusiform but often irregular and twisted, with periodical tapering towards the posterior end. It may be concealed by morphological aberrations. The metaboly of many species and the occurrence of dimensional varieties render the use of shape and size as specific features sometimes rather equivocal. Other features have to supplement them for the identification of the species.The shape of the nucleus is sometimes characteristic, but the structure of the plastidome and the presence or absence of pyrenoids are of much greater taxonomic importance. These features vary greatly in the species ofEnglena, while the other green genera have almost uniform chromatophores. By plastid and other characteristicsTrachelomonasandColaciumare related to certain species ofEuglena, whilePhacusandLepocinclisare nearer to others.The flagellar apparatus of the Euglenineae is tentatively considered as being composed of two flagellar units throughout the group, the length alone varying according to the genus.Eutreptiawith two equal or more or less unequal flagella would be nearest to the hypothetical ancestral form. The other green genera would be derived from it by a further shortening of the minor flagellum. Its near convergence to the long active flagellum gives the impression of bifurcation.Distigmawould be the apoplastidic counterpartof Eutreptia, Astasiathat ofEuglena.All green Euglenineae and some colourless ones have an eye‐spot and opposite to it a thickening of the active flagellum or flagella not found in the species without an eye‐spot.The striation of the periplast varies greatly, without correlation to the degree of cell metaboly. A first attempt at classifying the different types of metaboly is made. Envelopes differ from the periplast by being exudations of inorganic substances.While the distinction between the phototrophic and the saprotrophic Euglenineae seems gradually to disappear, the holozoic forms are more distantly derived. There are no indications as to their evolution, as no zootrophic green Euglenineae are known, and some are similar to saprotrophic forms. The entire class of the Euglenineae is rather uniform, and a rational and at the same time natural classification has not yet been att

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1948.tb00456.x

出版商:Blackwell Publishing Ltd    

年代:1948

数据来源: WILEY

摘要:

SummaryA brief account of some of the recent advances in our knowledge of the general biology, taxonomy and economic importance of the genusMerluccius, which have resulted mainly from the work of Hickling and of theDiscoveryinvestigations, has been given by way of introduction to a more detailed discussion of its distribution, and in the hope of demonstrating the great interest of hake from a general biological point of view.The seven species of true hakes (i.e. notUrophycisspp.) distinguished by Norman (1937), and their normal distributional limits, are:Merluccius merluccius(Linnaeus)From the Norwegian Rinne southwards along the edge of the continental shelf west of Europe to Dakar. Mediterranean.M. hubbsiMariniFrom Magellan Straits northwards overthe Patagonian continental shelf to southern Brazil.M. productus(Ayres) Southern California to north‐western Alaska. ? China.M. gayi(Guichenot) Southern Chile to Paita in Peru, and possibly farther north.M. bilinearis(Mitchill) South coast of Newfoundland southwards to North Carolina and, rarely (in deep water only) to Florida and the Bahamas.M. capensisCastelnauOff South Africa, from Angola to Natal.M. australis(Hutton)Chatham Island, South Island of New Zealand and northwards to East Cape on North Island. AChallengerspecimen from the Magellan region.There is a remote possibility that certain aberrant specimens secured in deep tropical waters off West Africa and in Panama Bay may represent two further distinct species, bridging the gaps betweenM. merlucciusandM. capensisand betweenM. gayiandM. pro‐ductus.It is more probable that they represent odd stragglers (at most only racially distinct) ofM. merlucciusandM. gayirespectively.It has been shown that all the best‐known species ofMerlucciusconform in a striking manner to the same distributional pattern in relation to the major hydrological features, within the limits of their normal range. Where relatively cold currents flow towards the equator in the warmer half of the normal habitat of any one of these species, the range of the species is extended in that direction; but if a relatively warm current is flowing polewards, the range in the direction of the equator is restricted. In the colder half of the normal habitat of each species the converse relationship holds good.Surface isotherms have been used as the most reliable general criterion symptomatic of the environmental complex that leads to this type of distribution because in many parts of the world more detailed hydrological data are not yet available; but it is emphasized that other factors, more or less intimately interrelated with the direct effect of temperature, are also involved.The distribution of the genusMerlucciusseems to offer a good example of the wider aspects of the phenomenon of ‘organic polarity’ discussed by Wimpenny (1941), while the bionomics of the better known individual species show some more detailed aspects of it with great clarity.I am greatly indebted to Dr N. A. Mackintosh, Director of Research,DiscoveryInvestigations, for permission to publish this review while I am still engaged on other work for theDiscoveryCommittee, and to Miss E. Humphrey, who re‐drew the chart for publication; also to the Director and staff of the Marine Biological Association's Plymouth Laboratory (where the article was written), especially the Librarian, Miss Sexton. Talks with Messrs Hickling, Carruthers, Deacon and Wimpenny have been most helpful and

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1948.tb00457.x

出版商:Blackwell Publishing Ltd    

年代:1948

数据来源: WILEY

摘要:

Summary1. Development in echinoderms may be indirect, involving pelagic, bilaterally symmetrical, larval forms, or more or less direct, with the larval stage either reduced or omitted. Of the five living classes, only the Echinoidea are characterized by being predominantly of the type with indirect development.2. It is possible to regard the dipleurula stage of the classes Asteroidea, Ophiuroidea, Echinoidea and Holothuroidea as recapitulating a common ancestralDipleurula.It is no longer possible to regard any of the other types of echinoderm larvae as anything but specialized forms without broad phylogenetic significance.3. Embryos and larvae of echinoderms are extremely plastic, often exhibiting convergence, divergence and adaptation, susceptible to evolutionary modifications of structure which may act quite independently of the adult stage. Ancestral structure cannot be deduced from such forms.4. In the Echinoidea larval evolution seems to have occurred subsequently to the separation of the main orders and families. Within relatively small groups larval evolution has followed similar trends, so that characteristic larvae occur in various sub‐groups, where the young stages may follow similar ontogenies; but such independent evolution tends to obscure the phylogenetic relationships between the class as a whole and the other classes.5. In the Asteroidea larval evolution has occurred along channels not so markedly correlated with the taxonomy of the adults. Phylogenetic speculations based on such larval stages prove incompatible with other evidence.6. In the Holothuroidea and Ophiuroidea larval evolution cannot at present be related with adult taxonomy, save in one or two cases too unimportant to have general significance.7. The egg of echinoderms is liable to undergo changes in volume. Increase of volume is directly related to increase in cytoplasm and its product, the yolk material. Such increases have led to direct development.8. Increase in cytoplasm and yolk has not greatly affected the cleavage process, which is almost always total. A distinction between micromeres and macromeres frequently results.9. With increasing cytoplasm, the wall of the blastula becomes thicker, and the blastocoel is in extreme cases reduced to a vestige in the animal hemisphere. The mesenchyme fails to separate as such, but projects as a solid mass into the blastocoel. Invagination is reduced to a solid inpushing of cells, and epiboly may ensue. The archenteron may become vestigial, in which case the definitive enteron is excavated in the solid endoderm by splitting. The enterocoele become reduced or lost, and the coelom and its adjuncts may arise by schizocoelous splitting in mesenchyme.10. In Ophiuroidea a succession of stages in reduction of the ophiopluteus may be seen, suggesting a recession backwards in time of the moment at which metamorphosis is initiated. In extreme cases the gastrula itself becomes radially symmetrical and the larva is completely lost.11. By convergent evolution among echinoderms with yolky eggs, a special vitel‐laria larva has arisen independently in Holothuroidea, Ophiuroidea and Crinoidea. The vitellaria is characterized by its barrel shape, and the transmutation of the ciliated band into annuli. In the Crinoidea this is the only larva as yet known.12. Viviparity does not seem to have been an important factor in causing direct development, though it may influence the physiology and morphology of the young stages.13. If larval stages of echinoderms are interpreted as recapitulating ancestral stages, the conclusions reached are seriously discordant with other evidence. Therefore it is not possible to base phylogenetic interpretations on larval stages alone.14. Echinoderm embryology cannot provide any valid support for the hypothesis that chordates arose from echinode

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1948.tb00458.x

出版商:Blackwell Publishing Ltd    

年代:1948

数据来源: WILEY