3. The internal layer of the blastodermic membrane forms the intestinal canal, which becomes lengthened and convoluted, and communicates with the exterior by a mouth and anus of secondary formation.

4. Finally the cerebro-spinal axis and its nerves, the skeleton, the organs of special sense, the integument, and the muscles, are developed from the external blastodermic layer; while the anterior and posterior extremities are formed from the same layer by a process of sprouting, or continuous growth.

CHAPTER VIII.

THE UMBILICAL VESICLE.

IN the frog, as we have seen, the abdominal plates, closing together in front and underneath the body of the animal, shut in directly the whole of the vitellus, and join each other upon the median line, at the umbilicus. The whole remains of the vitellus are then inclosed in the abdomen of the animal, and in the intestinal sac formed by the internal blastodermic layer.

In many instances, however, as, for example, in several kinds of fish, and in all the birds and quadrupeds, the abdominal plates do

Fig. 187.

not immediately embrace the whole of the vitelline mass, but tend to close together about its middle; so that the vitellus is constricted, in this way, and divided into two portions: one internal, and one external, (Fig. 187.) As the process of development proceeds, the body of the foetus increases in size, out of proportion to the vitelline sac, and the constriction just mentioned becomes at the same time more strongly marked; so that the separation between the internal and external portions of the vitelline sac is nearly complete. (Fig. 188.) The internal layer of the blastodermic membrane is by the same means divided into two portions, one of which forms the intestinal canal, while the other, remaining outside, forms a sac-like appendage to the abdoinen, which is known by the name of the umbilical vesicle.

The umbilical vesicle is accordingly lined by a portion of the internal blastodermic layer, continuous with the mucous membrane of the intestinal canal; while it is covered on the outside by a portion of the external blastodermic layer, continuous with the integument of the abdomen.

After the young animal leaves the egg, the umbilical vesicle sometimes becomes withered and atrophied by the absorption of its contents; while in some instances, the abdominal walls gradually

extend over it, and crowd it back into the abdomen; the nutritious matter which it contained passing from the cavity of the vesicle into that of the intestine by the narrow passage or canal which remains open between them.

Fig. 189.

In the human subject, however, as well as in the quadrupeds, the umbilical vesicle becomes more completely separated from the abdo men than in the cases just mentioned. There is at first a wide communication between the cavity of the umbilical vesicle and that of the intestine; and this communication, as in other instances, becomes gradually narrowed by the increasing constriction of the abdominal walls. Here, however, the constriction proceeds so far that the opposite surfaces of the canal come in contact with each other, and adhere; so that the narrow passage previously existing between the cavity of the intestine and that of the umbilical vesicle is obliterated, and the vesicle is then connected with the abdomen only by an impervious cord. This cord afterward elongates, and becomes converted into a slender, thread-like pedicle (Fig. 189), passing out from the abdomen of the foetus, and connected by its further extremity with the umbilical vesicle, which is filled with a transparent, colorless fluid. The umbilical vesicle is very distinctly visible in the human afth week. foetus so late as the end of the third month. After that period it diminishes in size, and is gradually lost in the advancing development of the neighboring parts.

In the formation of the umbilical vesicle, we have the first varia

tion from the simple plan of development described in the preceding chapter. Here, the whole of the vitellus is not directly converted into the body of the embryo; but while a part of it is taken, as usual, into the abdominal cavity, and used immediately for the purposes of nutrition, a part is left outside the abdomen, in the umbilical vesicle, a kind of secondary organ or appendage of the foetus. The contents of the umbilical vesicle, however, are afterward absorbed, and so appropriated, finally, to the nourishment of the newly formed tissues

CHAPTER IX.

AMNION AND ALLANTOIS.-DEVELOPMENT OF

THE CHICK.

WE shall now proceed to the description of two other accessory organs, which are formed, during the development of the fecundated egg, in all the higher classes of animals. These are the amnion and the allantois; two organs which are always found in company with each other, since the object of the first is to provide for the formation of the second. The amnion is formed from the external layer of the blastodermic membrane, the allantois from the internal layer.

In the frog and in fish, as we have seen, the egg is abundantly supplied with moisture, air, and nourishment, by the water with which it is surrounded. It can absorb directly all the gaseous and liquid substances, which it requires for the purposes of nutrition and growth. The absorption of oxygen, the exhalation of carbonic acid, and the imbibition of albuminous and other liquids, can all take place without difficulty through the simple membranes of the egg; particularly as the time required for the formation of the embryo is very short, and as a great part of the process of development remains to be accomplished after the young animal leaves the egg.

But in birds and quadrupeds, the time required for the development of the fœtus is longer. The young animal also acquires a much more perfect organization during the time that it remains inclosed within the egg; and the processes of absorption and exhalation necessary for its growth, being increased in activity to a corresponding degree, require a special organ for their accomplishment. This special organ, destined to bring the blood of the foetus into relation with the atmosphere and external sources of nutrition, is the allantois.

In the frog and the fish, the internal blastodermic layer, forming the intestinal mucous membrane, is inclosed everywhere, as above described, by the external layer, forming the integument; and