are developed in a fibrous membrane, with which the chorda dorsalis subsequently becomes invested; the neural arches being the parts first formed.

813. During the progress of this change, another very important one is taking place, which has reference to the nutrition of the embryo during its further development. This is the formation of vessels in the substance of the germinal membrane; which vessels serve to take up the nourishment supplied by the yolk, as well as that derived from the chorion externally, and to convey it through the tissues of the embryo. These vessels are first seen in that part of the vascular lamina of the germinal membrane, which immediately surrounds the embryo; and they form a network, bounded by a circular channel, which is known under the name of the Vascular Area (Fig. 142). This gradually extends itself, until the vessels spread over the whole of the germinal membrane. The vessels are probably formed by the coalescence of the original cells of the layer; and the first blood-discs which they contain seem to originate in the nuclei of these cells. This network of vessels serves to receive the nutritious matter contained in the yolk-bag, and to convey it to the embryo; but the act of absorption seems to be performed here as elsewhere, by cells, a layer of which always intervenes between the vascular layer and the yolk itself. These cells probably correspond in function with those of the villi of the intestinal canal in the adult (§ 243); as the vessels of the yolk-bag, or temporary diges

Fig. 142.

Vascular area of Fowl's egg, at the beginning of the third day of incubation; a, a, yolk; b, b, b, b, venous sinus bounding the area, e, aorta; d, punctum saliens, or incipient heart; e, e, area pellucida; f, f, arterios of the vascular area; 0, 0, veins; h, eye.

tive cavity, represent those of the alimentary canal, to be afterwards developed from a portion of it. The vessels of the yolk-bag terminate in two large trunks, which enter the embryo at the point that afterwards becomes the umbilicus, and which are known as the OmphaloMesenteric, Meseraic, or Vitelline vessels (Fig. 146, q, r). The first movement of fluid takes place towards the embryo; and this may be witnessed before any distinct heart is evolved.

814. The formation of the Heart takes place in a thickened portion of the Vascular layer; by the liquefaction of the interior of a mass of cells, of which the exterior constitute the first walls of the cavity. These gradually acquire firmness and consistency, and are endowed with a contractile power that enables them to execute regular pulsations. In this early condition, the heart is known as the punctum saliens (d, Fig. 142). The first appearance of the heart in the Chick is at about the 27th hour; the time of its formation in Mammalia has not been distinctly ascertained.

815. Concurrently with the formation of the Vascular system, the production of the permanent Digestive cavity commences. This originates in the separation of a small portion of the yolk-bag, lying immediately beneath the embryo, from the general cavity, in the following manner. At about the 25th hour of incubation, in the Fowl's egg, the parts of the germinal membrane which lie beyond the extremities, and Fig. 143.

Fig. 144.

Diagram of Mammalian Ovum at later stage; the digestive cavity beginning to be separated from the yolk-sac, and the amnion beginning to be formed:-a, chorion; b, yolk sac; c, embryo; d and e, folds of the serous layer rising up to form the Amnion.

The Amnion in process of formation, by the arching-over of the serous lamina:-a, the chorion; b, the yolk-bag, surrounded by serous and vascular lamina; c, the embryo; d, e, and f, external and internal folds of the serous layer, forming the amnion; g, incipient allantois."

which spread out from the sides of the embryo, are doubled in, so as to make a depression upon the yolk; and their folded edges gradually approach one another under the abdominal surface of the embryo, so as at last to meet and enclose a cavity, which is at first simple in its form, but which is subsequently rendered more complex by the prolongation and involution of its walls in various parts, so as to form the stomach and intestinal tube (Figs. 143, 144, 145). This digestive cavity communicates for some time with the yolk-bag (from which it has been thus pinched off, as it were), by a wide opening, that is left by the imperfect meeting of the folds of the germinal membrane that constitute its walls. In the Mammalia, this orifice is gradually narrowed, and is at last completely closed; and the yolk-bag, thus separated, is afterwards thrown off. It may be detected, however, upon the umbilical cord, up to a late period of pregnancy, and is known as the Umbilical vesicle (Fig. 146, t). In Birds, and other oviparous animals, the whole of the yolk-bag is

ultimately drawn into the abdomen of the embryo; the former gradually shrinking, as its contents are exhausted; and the latter enlarging, so as to receive it as a little pouch or appendage. In Fishes, the hatching of the egg very commonly takes place before the process has been completed; so that the little Fish swims about with the yolk-bag hanging from its body.

816. Whilst these processes are going on in the Vascular and Mucous layers of the germinal membrane, a remarkable change is taking place in that portion of the Serous lamina, which surrounds the Area pellucida. This rises up on either side in two folds (Fig. 143); and these gradually approach one another, at last meeting in the space between the general envelope and the embryo, so as to form an additional investment to the latter. As each fold contains two layers of membrane, a double envelope is thus formed; of which the outer layer (Fig. 144, d, e, and Fig. 145, h) afterwards adheres to the inner surface of the chorion, the original yolk-bag, or Zona pellucida, being now lost sight of; whilst the inner one (Fig. 144, f, f, and Fig. 145, ƒ) remains as a distinct sac, to which the name of Amnion is given. This takes place during the third day in the Chick; but the period at which it occurs in the Human Ovum has not yet been clearly ascertained.

817. The embryo, like the adult, has need of Respiration; in order that the carbonic acid set free in the Nutritive operations may be removed from its fluids. In Fishes, the surrounding water acts with sufficient power upon the vessels of the yolk-bag, to produce the required aeration, up to the time when the gills of the young animal are ready to come into play. But in the higher oviparous animals, whose development proceeds further before they leave the egg, a more special provision is made for the purpose. A bag, termed the allantois, sprouts (as it were) from the lower end of the body (Fig. 144, g); and gradually enlarges, passing round the embryo (Fig. 145, g), so as in Birds Fig. 145.

--h

Diagram representing a Human Ovum in second month:-a, 1, smooth portion of chorion; a, 2, villous portion of chorion; k, k, elongated villi, beginning to collect into Placenta; b, yolk sac or umbilical vesicle: c, embryo; f, amnion (inner layer); g, allantois; h, outer layer of amnion, coalescing with chorion.

almost completely to enclose it, intervening between the germinal membrane and the shell, and receiving the direct influence of the air that penetrates the latter. It is thus the temporary lung of the air-breathing

oviparous animal; and it serves for the aeration of its fluids, up to the time when it quits the egg. In the ovum of the Mammal, the chief office of the Allantois is to convey the vessels of the embryo to the chorion; and its extent bears a pretty close correspondence with the extent of surface through which the chorion comes into vascular connexion with the decidua,—this extent varying considerably in the different orders of Mammalia. Thus in the Carnivora, whose placenta extends like a band around the whole ovum, the allantois also lines the whole inner surface of the chorion, except where the umbilical vesicle comes into contact with it. On the other hand, in Man and the Quadrumana, whose placenta is restricted to one spot, the allantois conveys the foetal vessels to one portion only of the chorion (Fig. 146); although, according to Coste,

Fig. 146.

Diagram of the Circulation in the Human Ovum, at the commencement of the formation of the Placenta ; -a, venous sinus, receiving all the systemic veins; b, right auricle; b', left auricle; c, right ventricle; d, bulbus aorticus, subdividing into e, e', e", branchial arteries; f, arterial trunk formed by their confluence; g, vena azygos superior; h, confluence of the superior and inferior azygos; j, vena cava inferior; k, vena azygos inferior; m, descending aorta; n, n, umbilical arteries proceeding from it; o, umbilical vein; q, omphalo-mesenteric vein; r, omphalo-mesenteric artery, distributed on the walls of the vitelline vesicle, ; v, ductus venosus; y, vitelline duct; z, chorion.

it completely surrounds the embryo. When these vessels have reached the Chorion, they ramify in its substance, and send filaments into its villi; and in proportion as these villi form that connexion with the uterine structure which has been already described (§ 811), do the vessels increase in size. They then pass directly from the foetus to the chorion; and the allantois, which is no longer of any use, ceases to present itself as a distinct sac. The lower part of it, however, pinched off (as it were) from the rest, remains as the Urinary bladder; and the

Urachus or suspensory ligament of the latter represents the duct by which the Allantois was originally connected with the abdominal cavity. 818. The connexion which is thus formed between the Vascular system of the foetus and that of the parent, is the only one that exists in the lower Mammalia; which are thus properly designated as "nonpla cental." Each villus of the Chorion contains a capillary loop; this is enclosed in a layer of cells; and this again in a lamina of basementmembrane; the whole forming the fetal tuft. This comes into contact with the cellular decidua, which lies upon the basement-membrane covering the vascular layer of the decidua. Now the Placenta is composed of these very elements, arranged in a more complex manner. It is formed by an extension of the vascular tufts of the Chorion at certain parts; and a corresponding adaptation, on the part of the Uterine structure, to afford to these an increased supply of nutritious fluid. These specially-prolonged portions are scattered, in the Ruminants and some other Mammalia, over the whole surface of the Chorion, forming what are termed the Cotyledons; but in the higher orders, and in Man, they are concentrated in one spot, forming the Placenta. In some of the lower tribes, the maternal and foetal portions of the placenta may be very easily separated; the former consisting of the thickened decidua; and the latter being composed of the prolonged and ramifying vascular tufts of the Chorion, which dip down into it. But in the Human placenta, the two elements are mingled together through its whole substance.

819. The fatal portion of the Placenta consists of the branches of the umbilical vessels; which subdivide at the point at which they enter the mass, and form, by their minute ramifications, a large part of its substance. Each villus contains a capillary vessel, which forms a series of loops, communicating with an artery on the one side, and with a vein on the other; but the same capillary may enter several villi, before re-entering a larger vessel. The vessels of the villi (Fig. 147, g) are covered, as in the chorion, by a layer of cells (ƒ), enclosed in basement-membrane (e); but the foetal tuft thus formed is enclosed in a second series of envelopes (a, b, c), derived from the maternal portion of the placenta; a space (d) being left between the two, however, at the extremity of the tuft. The vascular tufts not unfrequently extend beyond the uterine surface of the placenta, and dip down into the uterine sinuses, where they are bathed in the maternal blood.-The maternal portion of the Placenta may be regarded as a large sac, formed by a prolongation of the internal coat of the great uterine vessels. Against the foetal surface of this sac, the tufts just described may be said to push themselves, so as to dip down into it, carrying before them a portion of its thin wall, which constitutes a sheath to each tuft. In this manner, the whole interior of the placental cavity, is intersected by numerous tufts of foetal vessels, disposed in fringes, and bound down by the membrane that forms its proper wall; just as the intestines are covered and held in their places by the peritoneum. Now as this dilatation of the uterine blood-vessels carries the decidua before it, every one of the vascular tufts that dips down into it will be covered with a