374 



CLASSIFICATION OF VERTEBRATES. 



the allantois early grows out to join the chorion, and brings 

 with it its blood-vessels, which then ramify through the chorion, 

 which therefore has its own circulation, although this is depen- 

 dent upon the allantois. In the ungulates the allantois, although 



well developed, re- 

 mains for a consid- 

 erable time distinct 

 from the chorion, and 

 only later, when its 

 expansion brings it in 

 contact with the lat- 

 ter, does the chorion 

 receive its vascular 

 supply. These two 

 types are known re- 

 spectively as the al- 

 lantoic and chorionic 

 placenta. 



In many mammals 

 the union between the 

 villi of the chorion 

 and the uterine walls 

 is slight, and at the 

 time of birth the two 

 separate, only the em- 

 bryonic placenta be- 

 ing cast off. These 

 forms — including the 

 ungulates, cetacea, si- 

 renia — are called non- 

 deciduata, or indecid- 

 uata. In others the 

 union is far more intimate, the branched villi entering into such 

 close connection with the uterus ' that, at the time of birth, a 

 portion or all the uterine walls (the decidua) is cast off with 

 the embryonic, or foetal, placenta. In some mammals, as in man, 

 the decidua exhibits certain peculiarities. At the time of at- 



FlG. 354. Diagram of human uterus and placenta, 

 based on Wiedersheim. Foetal parts lined, uterine 

 dotted, the decidual portions darker. A, cavity of 

 the amnion; F, foetal placenta; L, chorion laeve; R, 

 decidua reflexa; S, maternal placenta (decidua sero- 

 tina); T, entrance of Fallopian tube; T, decidua 

 vera. 



1 This forms the uterine placenta. 



