DEVELOPMENT OF BIEDS AND MAMMALS 251 



of the pit thus formed is met and fused with an amniotic fold 

 which, rising behind the tail and at the sides of the embryo, 

 extends forwards over the embryo. Before this has been 

 completed the allantois has appeared at the posterior end of 

 the enteron and carries the mesoderm out to the wall of the 

 serosa, around which it spreads. 



Internally the enteron, more and more defined by the 

 uplift of the embryo and its growth, takes on the features of 

 the adult alimentary canal, with others associated with em- 

 bryonic life and ancestral history. The stomodeum is developed 

 in front from the ectoderm, and expands inwards to meet with 

 the anterior end of the enteron. It gives off a hypophysis 

 as a dorsal outgrowth which comes into close association with 

 the anterior end of the notochord, and finally is attached to 

 the infundibulum to form the pituitary body. The pharynx 

 is early widened and gives off hyoid and two branchial clefts 

 which grow outwards on each side and are met by correspond- 

 ing folds of the ectoderm, but the clefts do not become open. 

 The arches are developed between the clefts. 



The rest of the alimentary canal is resolved into stomach 

 and intestine, and the glands are formed as outgrowths. 

 Posteriorly the neurenteric canal remains as a communication 

 with the neural canal during the early part of the develop- 

 mental period. Then it becomes solid and degenerates, leaving 

 for a time a short postanal gut. The proctodeum is formed 

 from the part of the primitive streak which is shut out from 

 the neural canal. 



The mesoderm, developed on each side of the notochord 

 and primitive streak, forms a double wing extending outwards 

 from the embryo over the yolk sac. In most cases it does 

 not advance beyond the upper hemisphere of the yolk sac. 

 It is divided into somatopleure and splanchnopleure, enclosing 

 the coelom, remaining joined at the margin and internally 

 in the neighbourhood of the notochord and the primitive 

 streak. The two wings extend forwards, as in the chick, and 

 fuse. The coelom thus extends widely around the embryo. 

 In the case of mammals which preserve the primitive amnion 

 the mesoderm passes into the space between the endoderm 

 and the serosa. As in the chick, the embryos of mammals 



