THE EMBRYO, 291 



the embryo from the rest of the ovum, so as to complete 

 its visceral walls, is effected by the folding-in of the 

 layers at the sides, by the prolongation backwards of the 

 folding-in which has been spoken of as occurring in the 

 head, and by a similar folding-forwards at the pelvic end 

 of the body, until at last the neck of communication 

 of the embryo with the rest of the ovum is narrowed to 

 the navel, where the vessels of the allantois pass in and out. 



Fig. 155. DEVELOPMENT or THE PALATE IN THE EMBRYO LAME, 

 in two stages more advanced than that exhibited in the previous 

 figure. From drawings by Professor Dickson. 



By this means both the cylinder of the visceral walls and 

 that of the alimentary tube are completed. For a time the 

 remains of the yelk sac continue connected with the alimen- 

 tary tube, which projects in a loop at the navel from the 

 completed wall of the abdomen, and they go by the name of 

 the umbilical vesicle, while the pedicle of connection is called 

 the vitelline duct ; it is not, however, a hollow pedicle; and 

 even in those classes of animals which have large yelk sacs, 

 the contents do not pass into the alimentary canal, but are 

 absorbed by the blood-vessels round it, for the nourishment 

 of the young animal. The place where the vitelline duct is 

 attached to the intestine, is near the lower end of the ileum. 

 As the walls of the embryo become folded in, the outer 

 layer of the germinal membrane, which is gradually expand 



