REPRODUCTION 



115 



Figs. 86 and 87 will serve better than description to make clear the rela- 

 tions of layers and spaces resulting from the development of these folds. 

 The somatopleural folds which give rise to the amnion and chorion are, 

 at the time of their formation, a living part of the embryo. The state- 

 ment that the folds eventually enclose the embryo anticipates the fact 

 that the amnion and chorion do not become any part of the adult. There- 

 fore "the embryo" which the folds enclose is the definitive body 



zdJX'- 



Fig. 85. — A, dorsal view of head of a chick embryo after about 33 hours incubation. 

 The head-fold of the amnion covers a small anterior portion of the embryo's head. 

 X32. B, section of head of an embryo at a similar stage, the plane of section indicated 

 by 5-5 in A. A, auditory invagination of ectoderm; AM, head-fold of amnion; EC, 

 ectoderm; EC, superficial ectoderm of head; EN, endoderm; H, heart; HE, endotheUal 

 cavity of heart; MH, hypomere mesoderm which gives rise to the myocardium, epicar- 

 dium and pericardium (see page 107); MS, mesodermal somites; NC, notochord; AT, 

 neural tube; O, optic vesicle; P, pharynx; i, fore-brain; 2, mid-brain; 3, hind-brain. 

 (Modified from figures by Duval, Atlas d'Embryologie.) 



region of the embryo. Everything else is conveniently referred to as 

 extra-embryonic. 



Since it is a fold of somatopleure which covers the embryo, there are 

 necessarily, at the completion of the process, two layers of somatopleure 

 external to the embryo. The inner one, the amnion, has its ectodermal 

 surface toward the embryo. The outer one, the chorion or serosa, has its 

 mesodermal surface toward the embryo. Between outer and inner 

 layers of the fold the mesoderm-lined space is obviously a part of the 

 coelomic cavity but, because it becomes no part of the adult coelom, it is 

 described as "extra-embryonic." 



The allantois is a sac formed as a mid-ventral outgrowth from the 

 hind or cloacal region of the embryonic enteron (Figs. 84, 86-88). It is 



