244 ESSENTIALS OF ZOOLOGY 



of the yolk continues by the endoderm, and as the embryo 

 increases in size the yolk thus invested appears as a sac depend- 

 ing by a neck of ectoderm and endoderm, each reinforced by 

 a layer of mesoderm, the ectoderm expanding into the amnion 

 and the endoderm investing the yolk. It is now called the 

 yolk sac, and the neck the umbilical cord. 



The allantois is developed as an outgrowth from the floor 

 of the hind-gut. The allantois, carrying the mesoderm with 

 it, grows into the splanchnocoel below the gut, invades the 

 umbilical cord, and expands into the extra-embryonic body 

 cavity to apply itself against the serosa. The mesoderm of 

 the allantois unites with that of the serosa, and the allantois 

 gradually spreads over the inner surface of the serosa first 

 above the embryo, but, extending downwards on all sides, it 

 separates the serosa from the endoderm of the yolk and finally 

 completely surrounds the serosa. It is, as we shall see, used 

 for respiration. 



Within the embryo the endoderm has in the meantime 

 been making important advances. Its anterior end is met 

 by an invagination of the ectoderm, which is the stomo- 

 deum, and which yields the hypophysis or pituitary body as 

 an outgrowth which becomes attached to the infundibulum. 

 Before the enteron and the stomodeum have been put into 

 communication by the breaking down of the double wall which 

 separates them, this front end of the enteron is expanded as 

 the pharynx and is produced laterally into a series of clefts. 

 These are the hyoid and the first, second, and third branchial 

 clefts, and in the walls separating them visceral arches are 

 developed. The clefts and the arches correspond exactly 

 with those of the fish and the amphibian, but they do not form 

 gill folds. They meet and fuse with corresponding ectodermal 

 invaginations which form distinct external grooves along the 

 neck region. They all disappear except the hyoid cleft, which 

 is retained to form the Eustachian tube and middle ear. 



The floor of the anterior part of the enteron is produced 

 into a pouch which becomes the thyroid. The thymus is 

 produced from the branchial clefts. The stomach and intestine 

 are gradually formed from the enteron as development proceeds, 

 and outgrowths give rise to the lungs and the glands. 



