The mouth and anus develop at a comparatively late period as pittings of the surface of the body, which gradually deepen until they meet at and open into the alimentary canal, which then first acquires its definite communications with the exterior. The mouth opening is formed before the anal one, which is usually not completed until very late in develop ment. In exceptional cases it may not be formed at the time of birth, thus giving rise to congenital occlusion of the rectum.
The whole embryo now grows rapidly, and about the twelfth day acquires the shape and appearance shown in Fig. 97, the rudiments of all the important organs being by this time established. The embryo is no. longer straight, but is very strongly bent on itself, the dorsal surface being convex and the ventral concave; the head end is disproportionately large, as, indeed, it is throughout the whole of development; in the neck a series of transverse ridges, the visceral arches, have appeared, and between these are slits, the visceral clefts, which lead into the alimentary canal, and so place it in communication with the exterior. Of the visceral arches the most anterior pair (Fig. 97, m x) lie at the sides of the mouth and form the basis of the upper jaw; the second pair (Fig. 97, in d) bound the mouth behind and give rise to the lower jaw; while the third par (Fig. 97, h y) form ultimately part of the hyoidean apparatus.
Rudiments of eyes (Fig. 97, op) and ears are present, as are also both pairs of limbs (Fig. 97, f 1, h 1) in the form of simple buds, presenting as yet little or no trace of subdivision into segments.
The Embryonic Membranes.
We have now to consider certain structures which, though not forming any actual part of the embryo itself, or only doing so to a very limited extent, yet play an exceedingly important part in its development. To gether with the formation of these embryonic membranes on the side of the ovum, it will be convenient to consider certain changes which occur in the uterus itself, and which are closely connected with the former.
the Decidua.
Previous to the arrival of the ovum, the lining membrane of the whole of the uterus has become much swollen and very vascular, and has un dergone other changes, which will be more fully described hereafter, lead ing to the formation of what is called the decidua, a Membrane specially developed to receive the ovum. The ovum on entering the uterus comes in contact with the decidua at some spot, and adheres to it; the decidua soon grows up around the ovum, forming at first a low wall round it, and ultimately completely enveloping it. The portion of the decidua which
grows over the ovum in this way receives the name decidua reflexa; the part with which the ovum first came in contact is the decidua serotina; and the part lining the rest of the uterus, the decidua vera; this latter is of no further use, and soon disappears; it is apparently only developed because it is uncertain with what portion of the uterus the ovum will first come in contact, and consequently all parts must be ready to receive it.
The ovum embedded in the decidua reflexa at first occupies but a small portion of the cavity of the uterus, but owing to its rapid growth it soon fills the greater part, and ultimately, in woman, the whole of the uterus, the decidua reflexa coming in contact with the decidua vera.
We now return to the ovum itself. At the stage represented in Fig. 98, 1, which is slightly older than that in Fig. 94, the ovum or blastoder mic vesicle is completely surrounded by the decidua reflexa, and has the following structure. The whole vesicle is invested in the zona pellucida, which gives off from its outer surface a number of little processes or villi (Fig. 98), which fit into little depressions in the decidua reflexa. The blastodermic vesicle itself presents in the embryonic area all three layers, 'epiblast, mesoblast, and hypoblast, and, as shown in the figure, the mesoblast has extended a little way beyond the embryonic area, so that surrounding this area is a ring in which, as in the area itself, all three layers are present. The rest of the vesicle consists of two layers, epiblast (or ectoderm of Kiilliker) and hypoblast (entoderm, Kiilliker), the latter having now grown so as to line the whole of the vesicle (ff. the earlier stage represented in Fig. 94, in which it has only extended about half way round).
Splitting of 3fesoblast.—The mesoblast (mesoderm, Kolliker) con tinues to spread by growth at its margin, and ultimately, like the hypo blast, extends completely round the vesicle. About the time of the first appearance of the constriction separating the embryo from the yolk-sac, the mesoblast splits into two layers. Owing to this splitting, which takes place in the marginal, though not in the axial part of the embryonic area, as well as in the part of the mesoblast outside the embryo, the portion of the blastodermic vesicle in which it occurs consists now of four instead of three layers—i.e., first, the epiblast; secondly, the upper or somatic layer of the mesoblast; thirdly, the lower or splanchnic layer of the meso blast; and, fourthly, the hypoblast.