340 



THE MAMMALIAN EMBETO. 



[chap. 



equivalent to that of a chick on the fourth day, re- 

 sembles in almost every respect the normal embryos of 

 the Amniota. The cranial flexure is as pronounced as 

 usual, and the cerebral region has now fully the normal 

 size. The whole body soon becomes flexed ventrally, 

 and also somewhat spirally. The yolk-sac (B; ys) forms a 

 small spherical appendage with a long wide stalk, and 

 the embryo is attached by an allantoic stalk with a 

 slight swelling, probably indicating the presence of a 

 small hypoblastic diverticulum, to the inner face of the 

 chorion. 



A detailed history of the further development of 

 the human embryo does not fall within the province of 



Fio. 113. 



Figures shewing the Early Changes in the form of the 

 Human Head. (Ftovi Qnain'a Anatomy.) 



A. Head of an embryo of about four weeks. (After 



Allen Thomson.) 



B. Head of an embryo of about six weeks. (After Ecker.) 



C. Head of an embryo of about nine weeks. 



1, mandibular arch ; 1'. persistent part of hyomandibular cleft ; 

 a. auditory vesicle. 



