126 



THE HUMAN EMBRYO. 



chorionic villus, and out of the neighboring chorionic ectoderm will be differen- 

 tiated the ectodermal covering of the villus. It seems, from a comparison of 

 later stages, that the trophoblastic degeneration never goes so far as to leave any 

 of the chorionic villi without an ectodermal covering. But this covering varies 

 extremely in its exact character as we find it in later stages, even in adjacent 

 parts of the same villus, for it may be either a single layer of cells or a layer of 



Am. 



Yk. 



Cho. 



Vi: 



Fig. 60. — Embryo of a Gibbon (Hylobates concolor) in the Third Stage. 

 Am, Amnion. Yk, Yolk-sac. Cho, Chorion. Vi, Villi. — [A/tcr E. Seltnka.) 



cells covered by a thin coat of syncytium or merely a syncytial layer (compare 

 page 342). The disappearance of all of the trophoblast, except so much as re- 

 mains to share in forming the ectodermal covering of the villi, produces the so- 

 called intervillous spaces of later stages, in which, as above stated, maternal 

 blood circulates. 



