208 



EMBRYOLOGY. 



product of the extra-embryonic part of the ectoderm and of the 

 parietal mesoderm united with it. 



The two infoldings, which lie near to each other, have opposite 



Fig. 125. Diagrammatic longitudinal section through the axis of an embryo Bird, after 



BALFOUR. 

 The section represents the condition when the head-fold is already formed, but the tail-fold i 



still wanting. 

 F.So, Head -fold of the somatopleure ; F.Sp, head-fold of the splanchnopleure, forming at Sp 



the floor of the anterior part of the intestine. For the remaining references see fig. 122, 



p. 201. 



directions (fig. 125). While the head-fold (F.So) advances with its 

 margin toward the yolk, the anterior fold of the amnion (Am), sepa- 

 rated from it by the marginal groove, rises externally above the 



Fig. 126. Diagrammatic longitudinal section through the posterior end of an embryo Chick at 



the time of the formation of the allantois, after BALFOUR. 

 tp, me, hy, Outer, middle, and inner germ-layers ; ch, chorda ; Sp.c, neural tube ; n.e, neurenteric 



canal ; p.a.g, post-anal gut ; pr, remains of the primitive streak folded toward the ventral 



side ; al, allantois ; an, point where the anus will be formed ; p,c, perivisceral cavity ; 



am, amnion ; so, somatopleure ; sp, splanchnopleure. 



plane of the blastoderm. At the time when the head is being formed r 

 the amnion enlarges rather rapidly (Plate I., fig. 1 1 vaj), and grows over 

 and around the head in a cap-like fold, the rim of which is directed 

 backwards. At the end of the second day of incubation it already 



