Fig. 138.

A

tion; and he found that when the interval of time between the one application and the other was only one or two seconds, only the early stages of the process took place, and no embryo was produced; when an interval of five seconds was allowed, very few embryos were produced from a large number of ova; but when the interval was fifteen seconds or more, the proportion of embryos produced was much greater. Thus it seems obvious that time is an important element in the fertilizing process; and that fertilization may be incompletely effected, for want of a sufficient penetration of the product of the diffluence of the Spermatozoa. How this product acts upon the contents of the ovum, however, and whether one or many of the cells set free by the rupture or solution of the germinal vesicle are fertilized by it, have not yet been ascertained. 805. The first change which can be observed to be consequent upon fecundation in the Mammalian ovum is the "segmentation" of the yolk; the entire mass of which, though previously compact and uniform, resolves itself, first into two, then into four, then into eight segments (Fig. 138); each segment containing a transparent vesicle, which may be surmised to be a descendant of the original germ-cell. By a continuance of the same process, the whole cavity of the vitelline sac, or zona pellucida, becomes occupied by spherical particles of yolk (each containing a pellucid particle), the aggregation of which gives it a mulberry-like appearance; and by its further continuance, the component cells becoming more and more minute, the mass comes to present a uniform finely-granular aspect. At this stage it does not appear that the several segments of the yolk have a distinct enveloping membrane; but an envelope is now formed around each of them, converting it into a cell, of which the included particle forms the nucleus. This happens first to the peripheral portions of the mass; and as its cells are fully developed, they arrange themselves at the surface of the yolk into a kind of membrane; at the same time assuming a pentagonal or hexagonal shape from mutual pressure, so as to resemble pavement epithelium (Fig. 139). As the globular masses of the interior are gradually converted into cells, they also pass to the surface and accumulate tion of the vitellus of the Mammalian there; thus increasing the thickness of the envelope already formed by the more super-ducing numerous segments. ficial layer of cells, while the central part of

C

B

Progressive stages in the segmenta

Ovum:-A, its first division into two

halves: B, subdivision of each half into two; C, further subdivision, pro

the yolk remains, filled only with a clear fluid, which seems to be the product of the liquefaction of some of the interior spherules. By this process, the external part of the yolk is converted into a kind of secon

dary envelope, constituting the germinal membrane; and as this forms a complete sac enveloping the liquefied yolk of the interior, and as the

Latter stage in the segmentation of the yolk of the Mammalian Ovum ;-at A is shown the "mulberry mass" formed by the minute subdivision of the vitelline spheres; at B, a further increase has brought its surface into contact with the vitelline membrane, against which the spherules are flattened.

whole structure of the future embryo originates in its substance, it has been termed by Bischoff the blastodermic vesicle.

806. The Blastodermic Vesicle very soon after its formation, presents at one point an opaque roundish spot, which is produced by an accumulation of cells and nuclei of less transparency than elsewhere; and it is within this, which is termed the area germinativa, that all the structures of the permanent organism originate. When seen in section, this mass of cells presents the aspect shown at Fig. 140, c. The Germinal Membrane increases in extent and thickness by the production of new cells; and it subdivides into two layers, which, although both at first composed of cells, soon presents distinctive characters, and are concerned in very different ulterior operations. The outer one of these is commonly known as the serous layer, and the inner as the mucous; the division is at first most evident in the neighborhood of the area germinativa; but it soon extends from this point, and implicates nearly the whole of the germinal membrane.

807. The area germinativa soon loses the rounded form which it at first possessed, and becomes first oval, and then pear-shaped. Whilst

Fig. 140.

Plan of early Ulerine Ovum. Within the external ring, or zona pellucida, are the blastodermic vesicle, a; the yolk, b; and the incipient embryo, c.

this change is taking place in it, there gradually appears in its centre a clear space, termed the area pellucida; and this is bounded externally

by a more opaque circle (whose opacity is due to the greater accumulation of cells and nuclei in that part), which subsequently becomes the area vasculosa. In the formation of these two spaces, both the serous and mucous layers of the germinal membrane seem to take their share; but the foundation of the vertebral column and nervous centres appears to be laid chiefly if not entirely in the serous layer; whilst the mucous is afterwards concerned more especially in the formation of the nutritive apparatus. Between these a third layer subsequently makes its ap-. pearance; which, as the first vessels of the embryonic structure are formed in it, is termed the vascular layer.

808. Thus the first development of the Mammalian embryo is into a sac, enclosing the store of nutriment that has been prepared for it,in fact, a stomach; and we shall presently see, that it is by the agency of the walls of this sac, that the nutrient materials which it encloses are prepared for being appropriated to the development of the more permanent part of the fabric, which is to be evolved from the centre of the mulberry mass. But we may here stop to notice the interesting fact, that the development of the ovum in the lowest classes of animals may almost be said to cease at this point; the external layer of the germinal membrane remaining as the integument; the internal layer becoming the lining of the stomach; and the space occupied by the yolk forming the digestive cavity, into which an entrance or mouth is formed, by the thinning away of the germinal membrane at a certain point, round which tentacula or prolonged lips are usually developed. This is the essential part of the history of development in the simpler Polypes; and we see how remarkably it corresponds with the history of development of the lower Cryptogamic plants, in which the first-formed membranous expansion, or primary frond, remains as the permanent leaf. In the Mammalia, on the other hand, the greater part of the germinal membrane, and of the cavity which it forms, have a merely temporary purpose; being cast off, when they have performed their function, like the cotyledons of Flowering Plants.

809. During the time which is occupied by these important changes, the Ovum passes through the Fallopian tubes, and makes its way into the Uterus. During its transit through the Fallopian tubes, the Mammalian ovum,-like the ovum of Birds in its passage through the oviduct, receives an additional layer of albuminous matter secreted from the walls of the tube; and this is surrounded by a fibrous membrane, whose structure and mode of formation have been described on a former occasion (§§ 181, 182). The outer layer of this envelope, in the egg of the Bird, is further consolidated by the deposition of particles of carbonate of lime in its areola; but it undergoes no higher organization. In the Mammal, however, this new envelope (termed the Chorion) is a formation of great importance; being the medium through which the whole subsequent nutrition of the embryo is derived. This is at first taken in by means of a number of villous processes, proceeding from the entire surface of the Chorion, and giving it a spongy or shaggy appearance; these processes (which are composed of nucleated cells) serve as absorbing radicles, which draw in the fluids afforded by the parent; and they thus make up for the early exhaustion of the small

supply of nutritious matter stored up in the ovum itself. The contained embryo appropriates the fluid which is thus imbibed, by simple absorption through its surface; and thus it is nourished until a more special provision for its development comes into action. The structure of this organ, termed the Placenta, cannot be understood, until the concurrent changes in the lining membrane of the Uterus have been considered.

810. This membrane, in its natural condition, presents on its free surface the orifices of numerous cylindrical follicles; which are arranged parallel to each other, and at right angles to the surface. In the spaces between these follicles, the blood-vessels form a dense capillary network. When impregnation takes place, this mucous membrane swells and becomes lax; its capillaries increase in size; the follicles are developed into glandular cavities, and become turgid with a white epithelium; and the interfollicular spaces are crowded with nucleated cells, which fill up the meshes of the capillary network. In this peculiar condition, the uterine mucous membrane is termed the Decidua. At a later period, the decidua may be found to consist of two distinct layers; the decidua vera, lining the uterus; and the decidua reflexa, covering the exterior of the ovum. Much discussion has taken place with regard to the mode in which the decidua reflexa originates, and the question cannot even now be considered as determined. The view recently put forth by Coste is, perhaps, as probable as any. He considers that the ovum on its entrance into the uterine cavity, is partly imbedded in its thick vascular lining membrane, and that this swells up around it, like the granulations around the pea in an issue; so that at last the ovum becomes completely invested by the special envelope thus formed, which closes in around it, constituting the decidua reflexa, and which is at first not in contact with the decidua vera at any part save where it has sprung from it. As the ovum increases in size, the decidua reflexa grows with it, and is thus gradually brought into contact with the decidua vera which lines the uterus, the cavity between them being obliterated; and at a later period, the two coalesce, so that they are no longer distinguishable from each other.

811. When the ovum has arrived in the Uterus, therefore, and the villous tufts of its chorion are developed, these come into contact, in the first instance, with the epithelial layer which intervenes between them and the vascular decidua. Through this cellular membrane, therefore, the ovum must derive its nutriment from the vascular surface; and it cannot be deemed improbable, that its office is to draw from the subjacent vessels the materials which are to serve for the nutrition of the ovum, and to present it to the villous tufts of the chorion. Each of these, as already mentioned, is composed of an assemblage of nucleated cells, which are found in various stages of development; and the villus seems to elongate by the development of new cells from the germinal spot at its free extremity, whilst, like the spongiole of the plant, it draws in nutriment from the soil in which it is imbedded. On the other hand, the Decidua at this early period, appears to be actively employed in preparing nutriment for the embryo; for its cellular layer is so abundant, as to form a bed into

which the tufts of the chorion are received; whilst its follicles are enlarged into glandulæ of sufficient size, to allow these villi (in some Mammals at least) to extend themselves into their interior.-In its earliest grade of development, as already remarked, the chorion and its villi contain no vessels; and the fluid drawn in by the tufts is communicated to the embryo, by the absorbing powers of its germinal membrane. But when the tufts are penetrated by blood-vessels, and their communication with the embryo becomes much more direct, the means by which they communicate with the parent are found to be essentially the same;-namely, a double layer of cells, one layer belonging to the foetal tuft, the other to the vascular maternal surface. (See § 819.)

812. We now return to the Embryo itself; the general history of whose development has been already traced, up to the period at which the cluster of cells in the Area Germinativa is about to give origin to the permanent structures of the foetus. The parts first formed in the embryo of Vertebrated animals, are such as most characteristically distinguish them from all others;-namely, the Vertebral Column, and Spinal Cord. The first indication of these consists in the formation of what is termed the primitive trace, which is a shallow groove, lying between two oval ridges (Fig. 141, v), known as the lamina dorsales. The form of these is changed with that of the area pellucida; at first they are oval, then pear-shaped, and at last become of a violin-shape. At the same time they rise more and more from the surface of the Area pellucida, so as to form ridges of higher elevation, with a deeper groove between them; and the summits of these ridges tend to approach one another, and gradually unite, so as to convert the groove into a tube. It is within this, that the Cerebro-spinal Axis is afterwards formed; the brain being developed in the anterior dilated portion, and the spinal cord in the posterior more contracted part. The

Fig. 141.

ag

The germ and surrounding parts, from a more advanced Uterine Ovum :-b, blastoderma, or germinal membrane; a g, area germinativa; c, cephalic extremity of the germ; v, first indications of vertebra; q, caudal extremity.

former remains unclosed much longer than the latter. The tube within which the neural axis is thus enclosed, and which is entirely composed of nucleated cells (Fig. 16), is termed the Chorda dorsalis; and it retains its embryonic type in many of the lower Fishes, which never possess a true vertebral column. The elements of the vertebræ