DEVELOPMENT 



627 



The orio^in of the first blood-vessels in the villi is not 

 known, but ultimately they are derived from the allantoic 

 arteries which pass out from near the posterior end of the hind gut. 



As the villi grow, the blood-vessels of the maternal mucosa 

 in the decidua serotina (fig. 241, D.S.) dilate, and the capillaries 

 form large sinuses or blood spaces. Into these the chorionic 

 villi pass, and thus the loops of foetal vessels hang free in the 

 maternal blood, and an exchange of material is possible between 

 the mother and foetus. 



The distribution of these villi is different in different 

 animals. In the horse they are diffusely scattered ; in the cow 

 and sheep they are arranged in circular patches, the cotyledons ; 

 in carnivora they are generally in a zone. 



Pig. 242. — Longitudinal Section through the Tip of a Villus of the 

 Placenta, covered by its trophoblast layer, and containing a loop of 

 blood-vessels, and projecting into a large blood sinus, /. V.S., in the 

 maternal mucosa. 



In the pig, horse, and in ruminants, the connection of the 

 foetal blood-vessels with the maternal structures is not very 

 intimate, and when the young are born the foetal part of the 

 placenta separates from the maternal part, which is thus not 

 shed. Hence such animals are called non-deciduata. 



In rodents, insectivora, apes, man, and carnivora, the associa- 

 tion is so intimate that at birth the maternal part of the 

 placenta is shed along with the foetal. Hence these are called 

 deciduata. 



In the mesoblast, through which the allantoic arteries pass 



