504 



URO-GEN1TAL SYSTEM. 



contact with the deep-seated surface of the second chorion, 

 which we have just studied. It extends over and substitutes 

 itself for this surface, or, at any rate, penetrates the whole 



Fig. 141. Development of the allantoi's and third chorion.* 



Fig. 142. Third choriou or vascular chorion.f 



of the outside of the egg between the external surface of the 

 amnios and the internal surface of the chorion (Fig. 141, 13, 



* o, Umbilical vessel in process of atrophy, a/, Allantols. 13, 14, Allantols 

 extending upon the internal surface of the second chorion. a, Cavity of the 

 amnion. (Kolliker, "Entwickelungsgeschichte.") 



t a, Amniotic cavity, well developed, o, Umbilical vesicle, almost com- 

 pletely atrophied, al, Allantold vesicle, properly speaking. 15, Its vascular 

 villi completely developed, and forming the third chorion or vascular chorion 

 around the egg (see explanation of Fig. 139 for the distinction of the dark, in- 

 terrupted, and dotted lines). (Kolliker, "Entwickelungsgeschichte.") 





