THE HUMAN FCETAL MEMBRANES. 



39 



Reflected amnioti' 



Medullary folds. 

 Medullary groove- 



Neurenteric canal 



Primitive streak' 



Belly-stalk 



Chorion 



Chorionic villi^ — 



/ 



'/ 



VJr 



. by slightly stretching the umbilical cord at its insertion, when a fold appears 

 containing no large vessels. This fold points to the position of the yolk-sac. 



To sum up, the chief peculi- 

 arities of the human foetal mem- Fig. 45. 

 branes are the following: ■-. -^-- 



1. The amniotic cavity is Vitelline sac- 

 developed at a very early period 

 apparently by a process of hollowing 

 out in the region of the cells of the 

 inner mass, and not by any folding 

 process. The cells above this primi- 

 tive amniotic cavity are later split 

 into two portions by the entrance of 

 the mesoblast and extra-embryonic 

 coelom ; the inner portion becomes 

 the ectoblast of the amnion, the 

 outer portion is merely a part of the 

 the trophoblast of the chorion. 



2. The mesoblast and extra- 

 embryonic coelom are precociously 

 developed at a very early period. 



3. The body-stalk constitutes 

 a primary and permanent connection 

 between the embryo and the chorion. 



4. The allantois, which, ex- 

 ternal to the body of the embryo, is 

 an insignificant structure, grows 

 into the body-stalk and therefore is 

 never a free vesicle. 



5. The trophoblast is very early greatly proliferated and very early in intimate 

 contact with the maternal blood. 



Fig. 46. 



Ectoblast 



Mesoblast, 







Dorsal surface of early human embryo, two millimetres in 

 length. X 23. {After Spee.) The amnion has been divided 

 and turned aside. 



Vitelline sac 



Wall of vitelline 

 sac 



-Amnion 

 -Amniotic sac 



— Gut-tube 



Belly-stalk 



Chorionic 

 villi 



Neurenteric 

 canal 



Primitive 

 streak 



Chorion 



Longitudinal section of human embryo represented in preceding figure. X 23. (After Spee.) 



Fig. 44, page 38, is a reproduction of the drawing of Peter's embryo and 

 deserves special attention. The figure shows a small portion of the mucous mem- 

 brane of the uterus in which is imbedded the embryonic or chorionic vesicle. 



