446 OUTLINES OF CHORDATE DEVELOPMENT 



derm, and as we have seen, extend into or across the inter- 

 villous cavity from all surfaces of the blastodermic vesicle. 

 As the vesicle enlarges the villi on the sides toward the decidua 

 capsularis disappear along with the capsularis itself, and the 

 smooth chorion thus left is the chorion Iceve, which then comes 

 into contact with the decidua vera on the opposite side of the 

 uterus (Figs. 182, 183). The villi in relation with the decidua 

 basalis alone remain, forming then the chorion frondosum; 

 this part of the chorion is in the region of the attachment of 

 the body stalk of the embryo, where the umbilical (allantoic) 

 blood vessels are distributed. 



The villi of the chorion frondosum enlarge and branch, many 

 of them finally assuming a dendritic appearance, extending 

 irregularly through the intervillous cavity (Fig. 184). While 

 at first simply ectodermal, the mesoderm of the chorion soon 

 pushes out into them and becomes extremely vascular. Even 

 before this the superficial cells of the trophoderm (and villi) 

 have fused into a syncytial layer known as the syncytiotropho- 

 derm, which is lined internally, for a time, by a simple epithe- 

 lium of the. cells of Langhans, also apparently derived from 

 the chorionic ectoderm. The vascular mesoderm then forms 

 the core of the villus (Fig. 184). 



As the villi grow out they continue to erode the substance of 

 the decidua basalis, and the intervillous cavity is consequently 

 enlarged and filled with maternal blood from the opened 

 capillaries and vessels of the region. Finally the whole inter- 

 villous space is occupied by large vascular sinuses or lacunae, 

 and when the Langhans cells disappear, the embryonic and 

 maternal blood streams are separated only by the endothelium 

 of the villous capillaries and the syncytiotrophoderm covering 

 them (placenta hsemochorialis). Of course the two blood 

 streams are nowhere in direct communication. 



The formation of the placenta has now been described. It 

 is a structure combined from two distinct sources, a maternal 

 portion, the decidua basalis, and an embryonic portion, the 

 chorion frondosum; the two are separated by the intervillous, 

 or as we may now say, intra-placental cavity, filled with maternal 



