596 THE UMBILICAL VESICLE. 



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 



not immediately embrace the whole of 

 Fig. 204. 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. 204.) As the 

 process of development proceeds, the body 

 of the foetus increases in size, out of pro- 

 portion to the vitelline sac, and the con- 



EGG OF FISH, showing forma- r 



tiou of umbilical vesicle. striction 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. 205.) 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 abdo- 

 men, 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 por- 

 tion of the external blastodermic layer, continuous with the integu- 

 ment of the abdomen. 



