AVES 319 



The medullary plate and medullary groove forms much as in the 

 frog, beginning at the anterior end and proceeding to close gradually 

 from the anterior toward the posterior. 



While the axial parts of the embryo are differentiating the periph- 

 eral parts of the blastoderm continue to grow round the yolk, new 

 cells being continually formed at the margin. Finally the whole 

 ovum becomes covered with cells. A considerable part of this sheath 

 of cells is destined to be used only for the formation of embryonic 

 membranes — amnion, allantois and yolk-sac. Only the .parts near 

 the animal pole of the egg are concerned in forming the embryo 

 proper. The embryo is gradually pinched off from the rest of the 

 egg by means of deep grooves that go so deep as finally to leave only 

 a narrow yolk-stalk between embryo and yolk. An extensive vitelline 

 circulation covers the yolk sphere, and through this means the embryo 

 maintains a nutritive connection with the yolk. 



Like all other vertebrates the young chick (Fig. 163) develops 

 four pharyngeal clefts (gill-slits), only one of which actually breaks 

 through to the pharynx; this is the eustachian tube of the adult. It 

 has been commonly stated that the bird embryo never exhibits any 

 traces of gill filaments in these gill-slits, but Boyden has recently de- 

 scribed not only in the chick but in several reptiles the transitory 

 appearance of tissues, which he believes are undeniably rudimentary 

 branchial filaments. 



Embryonic Membranes. — The importance of the amnion and 

 allantois (Fig. 164) as adaptations for land life, and their role in the 

 evolution of the terrestrial vertebrates, have been sufficiently dealt 

 with in the chapter on reptiles. In general the mode of origin of 

 these membranes is the same in the bird as in the reptile. The amnion 

 begins as a crescentic fold of the extra-embryonic blastoderm in front 

 of the head. This fold, which consists of ectoderm and mesodern only, 

 grows backwards, covering the head like a hood and continues to 

 spread over the body until it meets a smaller, but similar tail fold that 

 has been growing forward. The two folds fuse together and com- 

 pletely inclose the embryo in a sac lined with ectoderm on the in- 

 side and mesoderm on the outside. Of course an outer section of the 

 fold is also produced, called the chorion, which is lined with ectoderm 

 on the outside and with mesoderm on the inside. Thus two complete 

 membranes shut off the embryo from the albuminous layers. The 

 inner layer of the amnion secretes an abundant watery fluid that 



