vascularization by arteries and veins is required for the nutrition of the new being undergoing process of development. An increase of the muscular elements is required for the process of expulsion (parturition, or labor) of the new being when perfectly developed (foetus at full term of pregnancy). It is sufficient to state that this act, like all those heretofore

studied, is under the influence of the nervous system; we shall see here also reflex phenomena analogous to all those which concern expulsion and excretion. The point of origin for these phenomena is normally in the uterus; but various excitations

* Vertical section of the womb, containing a developed egg or ovum. a, Neck, filled with a gelatinous plug. bb, Orifice of the Fallopian tubes. cc, Membrana decidua vera. d, Uterine cavity, almost entirely filled with the ovum. ee, Where the decidua vera is continuous with the decidua reflexa. f, Caduca serotina or placenta. g, Allantois. h, Umbilical vesicle and its pedicle in the umbilical cord. i, Amnion. k, Decidua reflexa and chorion.

can occasion it in parts even at a distance from the pelvic organs. Certain investigations on rabbits (W. Schlesinger) show that excitations of the central portion of the spinal nerves induce uterine contractions; and the same effect has been caused by excitations of the central portion of the pneumo-gastric; moreover, clinical observation has shown that a mechanical irritation of the breasts favors uterine contractions.

II. DEVELOPMENT OF THE FECUNDATED EGG.

The result of fecundation in the ovum consists of segmentation of the vitellus. We commenced our studies with the globular proliferation (p. 10); this is a type of one of the manifestations of the general characteristics of the globules, consisting of segmentation and reproduction. Simple segmentation can sometimes occur without fecundation; but, generally, the presence of the spermatozoids seems to set in action a physiological excitation which induces the division of the vitelline protoplasm; in every case of segmentation of the unfertilized ovum, this segmentation does not extend very far, and never forms the blastodermic membrane.

I. Envelopes of the embryo, respiration, nutrition.

These envelopes vary according to the period of the development of the embryo; and, since they are the seat of the exchanges between the foetal organism and its external medium (maternal organism), the manner in which these exchanges (nutrition and respiration) are performed depends upon the different periods of the embryonic life.

First, After the fecundated egg has traversed the tubal canal, and segmentation of the vitellus has occurred, the egg has no other envelope than its vitelline membrane (see Fig. 137) upon whose surface little homogeneous villi are developed; these constitute the first chorion (Fig. 137, 1). By the process of osmosis and imbibition, the albuminous liquids in the Fallopian tube and the uterine cavity pass through this membrane, and are borne along with the segmentation of the vitellus.

Secondly, when the segmentation is completed, and the blastoderm is formed, the relations between parent and embryo are more regularly established by the formation of new envelopes and a placenta; but, in the human species, at this transitional period, there is established a mode of nutri

tion, which is more durable than in the ovipara, which has its source from an organ called the umbilical vesicle; finally, when the body of the embryo is developed it is protected in

Fig. 137. -Commencement and development of the egg.*

a pouch or diverticulum, amnion, whose fluid contents ward off sudden compressions.

Umbilical vesicle. When the blastoderm (see p. 17) is formed around the egg, by its simple nutrition, as above shown, it attains a considerable size, from the fact that its interior forms a cavity, whilst the division of the blastoderm into three folds (u, a) becomes more pronounced at that place in which the body of the embryo will be formed (Fig. 138). But as the embryo becomes gradually developed, the circular region, by which it forms a part of the common blastodermic vesicle, gradually recedes (from 9 to al, Fig. 139) in such a manner, that soon the primary cavity becomes divided into two secondary cavities (Fig. 137, o and 12), one of which forms a portion of the embryo (12), its future intestinal cavity (see pp. 184, 232, and 424), and the other forms a vesicle placed above the ventral portion of the embryo (Fig. 137, o), the umbilical vesicle, which communicates with the intestine only by a canal called the omphalomesenteric duct (Fig. 139 and 140); the place at which this

* 1, Vitelline membrane. 2, External layer of the blastoderm. 3, Middle layer. 4, Internal layer of the blastoderm. 5, Form of the embryo. 6, Cephalic fold of the amnion. 7, Caudal fold of the amnion. 8, Extremity of the cephalic fold, which tends to join the corresponding extremity of the caudal fold. 9, Point for formation of heart. 10, Umbilical vesicle, yolk sac. 12, A portion of the internal fold of the blastoderm, from which the intestine will be formed.

duct is continuous with the intestine is the intestinal umbilicus, and the walls of the body which close around this duct form the cutaneous umbilicus, or navel (see p. 231, Fig. 64).

The umbilical vesicle contains a fatty albuminous fluid which represents all the extra-embryonic portion of the vitellus. This liquid serves for the nutrition of the mammalian fœtus until the placenta is developed. The absorption of the fluid of the umbilical vesicle is accomplished by a system of blood-vessels (primary circulation, see farther on),

which are developed in the external wall of the vesicle (omphalo-mesenteric vessels); these absorb the contents of this cavity by the medium of the internal epithelial surface of the vesicle, just as in adult life the mesenteric vessels (vena porta) absorb the contents of the intestinal canal by the medium of the villous epithelium, (and indeed fine vascular villi are often found on the internal surface of the umbilical vesicle).

*D, Yellow, 8, Vitelline membrane. u, Membrane at the external layer of the blastoderm. a, Middle layer. y, Internal layer.

† 1, Vitelline membrane. 2, External layer of the blastoderm. 3, Middle layer of the blastoderm. 4, Internal layer. 5, Body of the embryo. 6, 7, 8, 9, As in Fig. 137. o, Umbilical vesicle. al, Allantoid pouch or protuberance. a, Amniotic cavity.

In this Figure, as in Figs. 137, 140, 141, the interrupted lines show the parts belonging to the internal layer of the blastoderm; the black lines belong to the middle layer; the dotted lines belong to the external layer.

Yet, the existence and functions of the umbilical vesicle, or yolk sac, do not continue for any great length of time in the mammalia. The nutritive pabulum enclosed by it is not large; even at the fourth week the umbilical vesicle begins to atrophy, and towards the fifth only a trace of it remains (Fig. 142). In the ovipara, however (especially in birds), the umbilical vesicle lasts much longer, and plays a more important part in the nutrition of the chick; it contains the yellow substance, a provision which is sufficient for the development of the chick in the egg, and feeds it even after the chicken is hatched, but at that time it is enclosed inside the abdominal cavity, until the chicken is able to feed himself.

Amnion. As soon as the umbilical vesicle and the body of the embryo have been completely separated by the strangulation that we have already studied (intestinal and cutaneous umbilicus), the distinction between the three layers of the blastoderm becomes more and more complete, and the external one gives rise to a particular formation, the amnion and secondary chorion. In fact, as soon as the cutaneous umbilicus is formed, and at the same point, the external fold (cutaneous) of the blastoderm extends in such a way as to surround the embryo, and forms two lateral layers which tend to unite at its dorsal region, forming at the two extremities two hood-like folds (cephalic and caudal, Fig. 137, 6, 7) which cover its caudal and cephalic portions. Only the middle portion of the back of the embryo remains uncovered, but soon these folds and layers by process of development unite (Fig. 137, 8), until the only opening (amniotic umbili cus, Fig. 139, 8) is circumscribed and completely closed. From this time the embryo is enclosed in a cavity, amniotic cavity (Fig. 139, a), in which it is suspended in the ambient fluid, amniotic fluid, given out from the walls which form this cavity.

The internal surface of the amniotic cavity is formed by that entire portion of the external layer of the blastoderm, which has been isolated from the rest of this fold by the successive hood-like covering of the embryo and the union of the amniotic umbilicus. This surface is covered by an epithelial layer given off from a layer of embryonic connective tissue (from the middle fold), in which smooth muscular fibres may be seen (Fig. 140, 141, dark and dotted lines). On account of this formation the rest of the external fold of the blastoderm is henceforward completely isolated from the body of the embryo, and forms an extended envelope sub