Book - Vertebrate Embryology (1913) 1
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Jenkinson JW. Vertebrate Embryology. (1913) Oxford University Press, London.
- Vertebrate Embryology 1913: 1 Introduction | 2 Growth | 3 The Germ-Cells, their Origin and Structure | 4 The Germ- Cells, their Maturation and Fertilization | 5 Segmentation | 6 The Germinal Layers | 7 The Early Stages in the Development of the Embryo | 8 The Foetal Membranes of the Mammalia | 9 The Placenta | Figures
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Chapter I Introduction
Embryology is the study of the development of the individual organism - that is to say, of that long and frequently complex series of changes whereby from a relatively simple germ there is produced a new individual which, within the limits of ordinary variation, is like the parents that give it birth. Development, in other words, is the production of the form characteristic of the species ; it is the mechanism of inheritance. The startingpoint of the process may be a single cell, \^hich is either a fertilized ovum or at any rate the product of the union of two cells, as in all cases of sexual reproduction, or else an unfertilized egg-cell, as in parthenogenesis. Again, the ' germ ' from which the new organism is to arise may be a multicellular body derived from one or more of the tissues of the parent, as in budding. Lastly, in regeneration, or the replacement of lost parts, Avhere a new whole form is produced over again from a part of the original - whether that part is large or small - the process starts from a multicellular and a differentiated structure.
As a matter of common practice, however, the term ' development ' is often restricted to the first of these processes, and in the Vertebrates with the greater justification in that reproduction by budding does not occur in the group, although regeneration does. Neither has natural parthenogenesis ever been observed. Reproduction, then, in Vertebrates means sexual reproduction, and the developing individual springs from the union of two germ-cells.
These germ -cells are the vehicles whereby the inheritable characters of the species are handed on from one generation to the next ; they arc the material basis of inheritance.
In the study of this process two methods are at our disposal. Either we may content ourselves with a description of the series of changes which the ovum passes through, or else we may add experiment to observation in the attempt to discover the causes of each stage in the chain of events, and so of the whole. In the present treatise we shall limit ourselves to the former of these two inquiries.
In every development there are involved three kinds of activity - growth, cell-division, and differentiation.
Growth is increase of size, more properly of mass.
Cell-division, preceded always by karyokinetic division of the nucleus, is the first, or nearly the first, sign the fertilized ovum gives of its activity, and continues throughout the period of development, indeed throughout life itself, though at a diminishing rate.
But these are, relatively speaking, side issues. The problem about which interest really centres is the problem of differentiation, or increase of structure. The egg has indeed a structure, but that structure is not the structure of the parent that produced it, nor of the offspring to which it will give rise. It is more simple, and in development structure is increased, the simple gives way to the complex.
The process takes place in a series of stages which follow upon one another in regular order and with increasing complexity. When segmentation has been accomplished certain sets of cells, the germ-layers, become separated from one another. Each germ-layer contains within itself the material for the formation of a definite set of organs- the endoderm, for instance, contams the material for the alimentary tract and its derivatives-giUsHts, lungs, liver, bladder, and the like. The germ-layers are, therefore, not ultimate but elementary organs, and, elementary organs of the first order.
In the next stage these elementary organs become subdivided into secondary organs-the ectoderm is portioned into epidermis, sense-organs, and nervous system-and in subsequent stages these again become successively broken up into organs of the third and fourth orders, and so on, until finally the ul imate organs and tissues are formed each with special histological
characters of its own, as seen in the arrangement, shape, aiid size of the cells, structure of the nucleus, structure of the cytoplasm, and nature of the substances secreted by the latter, whether internally, as, for example, the contractile substance of muscle fibres, or externally, as in the matrix of bone. This end is, however, not necessarily reached by all the tissues at the same time. Indeed, it is no uncommon thing for certain of them to attain their final structure while the others are yet in a rudimentary condition. Vacuolated notochordal tissue, for instance, is differentiated in the newly-hatched tadpole of the frog, and, speaking generally, larval characters are developed at a very early stage.
Regular sequence of events, then, is one of the features of ontogeny, or the development of the individual, and another is composition, since the organs of the body are by no means formed of single tissues - bone, epithelium, blood, and the rest - but are compounded, often of very many.
While, therefore, in the last resort all differentiation is histological, that final result, the assumption by the cells of their definitive form, is only achieved after many changes have taken place in the position of the parts relatively to one another while the organs are being compounded, and so its specific shape being conferred upon the whole body.
But manifold though the changes are that occur in the relative position of the parts, they may all be embraced in a comparatively few general expressions, relating to the movements of single cells, or of cell-aggregates.
I. Movements of Single Cells Amongst movements of single cells are comprised :
- The migration of free amoeboid cells, for example, the lower layer cells in the blastoderm of Elasmobranchs, and the wanderings of the germ -cells in early stages.
- The aggregation of isolated cells :
- Linear aggregates, as in the formation of capillaries.
- Superficial aggregates, as in the formation of the yolk-sac of certain Mammals.
- Massive aggregates, as in the spleen.
- The attachment of isolated cells to another body, as in the union of tendon to bone, or the application of skeletal cells to the notochord.
- Investment and penetration by isolated cells, as in the septa of the corpus luteum, the cells which secrete the vitreous humour in the eye.
- Absorption by wandering cells, as in the phagocytosis of the tadpole's tail during metamorphosis.
- To these we may add here the frequent movements involving merely change of shape, as when fiat cells become columnar, or when a nerve fibre grows out from a nerve-cell.
II. Movements of Cell Aggeegates
A. Linear Aggregates
- Growth in length, as in the back growth of the segmental duct.
- At the end, that is, branching, for example, of nerves, blood-vessels, kidney-tubules, glands.
- Throughout the length ; for instance, the segmental duct of Elasmobranchs, the truncus arteriosus of Mammals.
- Anastomoses, as of nerves in the formation of nerveplexuses, of capillaries, of the bile-capillaries of the liver.
- Fusion with other organs : of a nerve, for instance, with its end organ, of the vasa efferentia with the tubules of the mesonephros.
B. Superficial Aggregates
- Increase in area, of a curved or of a plane superficies, as in the growth of the Mammalian blastocyst, or in that of the auditory vesicle, or of the medullary tube, or of the blastoderm over the yolk. When this growth is not equal in all parts of the surface the result is a local outgrowth or ingrowth-that is, an evagmation as in the outgrowth of the optic vesicles, or of the cerebral hemispheres, or an invagination, as in the formation of the medullary groove, or of the lens of the eye.
- Alterations of thickness, by increase, as in the formation of the placenta from the trophoblast in MammaUa, or by decrease, as in the roof of the thalamencephalon and medulla, or m the outer wall of the lens.
- Interruptions of continuity by the atrophy of part of a layer, as in the disappearance of Rauber's cells in certain Mammalian embryos, or in the perforation of the floor of the archenteron in Amniota, or by the detachment of a part of the layer, as when the notochord is lifted out of the archenteric roof in Urodela and Petromyzon.
- Concrescence of layers, as in the union of the embryonic plate vnth. the trophoblast in some Mammals, where the layers unite by their, margins, or as in the union of the medullary folds, or of the stomodaeum with the gut, where the concrescence is by the surfaces. In the latter case the cavities on opposite sides of the adherent layers commonly open into one another, as when the stomodaeum opens into the gut, or the amnion folds unite ; but not necessarily, as when the somatopleure fuses with the trophoblast, or the allantois with the somatopleure.
- SpUtting of a layer into two, for example, in the inner wall of the pineal vesicle in Lacertilia.
C. Massive Aggregates
- Changes in volume and shape, as in the outgrowth of limbbuds.
- Rearrangement of material, as in the formation of the concentric corpuscles of the thymus, or in the development of kidney tubules in the metanephric blastema of Amniota ; or again, when internal cavities are formed, such as the segmentation cavity, lumina of ducts and blood-vessels, of the coelom ; or, lastly, by the dispersion of the cell-elements of an aggregate, as in the liberation of the germ-cells.
- Division of masses, as in the metameric segmentation of the mesoderm and neural crest, or the separation of the (solid) nervous system from the ectoderm in Petromyzon, and Teleostei.
- Fusion of masses, as in the union of originally separate ganglia.
- Attachment of one mass to another, as of sclerotome to notochord.
Differentiation then takes place by these various movements of cells and of cell aggregates, and by the final assumption by the cells of the histological characters appropriate to each tissue.
The cells all arise from the continued subdivision of one original cell, the fertilized ovum.
But while this process of division is apparently necessary for development, it must not be supposed that it is the division which brings about the differentiation, for the simple reason that some differentiation already exists in the ovum before segmentation begins. Indeed, as we shall see in the sequel, the ovum is no homogeneous mass, but a heterogeneous body, provided with a definite structure, and this initial structure is the real cause of the differentiations that subsequently arise. A scientific account of development must therefore begin with the structure of the germ-cells.
For convenience' sake, however, we may first discuss very briefly the chief features of the phenomenon of growth.
C. B. Davenpoet. Studies in morphogenesis : iv. A preliminary catalogue of the processes concerned in ontogeny. Bull. Harvard Mus. xxvii, 1896.
J. W. Jenkinson. Experimental Embryology. Oxford, 1909.