FCETAL ENVELOPES 



383 



of connective tissue ramified by numerous blood-vessels and by 

 branched glands which are merely diverticula of the epithelium 

 Uning the cavity of the uterus (fig. 405). 



Except in the monotremes, the eggs, fertilized in the Fallopian 

 tube, descend into the uterus and there become implanted, either in 

 the cavity or actually enveloped in the mucosal wall. In the mar- 

 supials and in some other forms the yolk sac is large and nearly en- 

 closes the whole embryo and its amnion, and so leaves but a small 

 space where the allantois could possibly reach the chorion. The yolk 



Fig. 405. — Diagram of uterus simplex. /, opening of Fallopian tube; g, uterine 

 glands; m, muscular layer (no attempt to indicate direction of fibres); tnu, mucosa; 0, 

 early implanted ovum. 



sac is highly vascular and it becomes thrown into a number of folds 

 which become inserted into corresponding folds of the uterus, the 

 epithelial walls of which degenerate into a syncitial layer. By this 

 vitelline placenta, some nouishment is transferred to the young. 

 Among the marsupials, only in Perameles and Dasyurus does the 

 allantois reach the chorion, this part now resting against the uterine 

 syncitium, these two species having a true or allantoic placenta. All 

 other mammals have the allantoic placenta, the details of its structure 

 differing greatly in the separate orders and even within the same 

 order. There is also much intergrading, but the following general 



