KEPEODUCTION 437 



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. 



B 



C A 



FIG. 174. Schematic section of one cornu of the uterus of a ruminant at an 

 early stage of gestation to show the elongated umbilical vesicle, A, and 

 allantois, B, and the embryo in the amniotic sac, C. 



In Eodents, 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. 



2. Attachment to the Mother 



The ovum gets enclosed in the uterine mucous membrane, 

 which grows round it as the deeidua reflexa (fig. 175, D.R.). 



Almost as soon as the ovum is embedded in the maternal 

 mucous membrane, it becomes surrounded by a nucleated mass 

 of protoplasm the trophoblast, formed of the cells of the 

 ectoderm, and this probably transfers nourishment from the 

 mother to the ovum. Very early the mesoblast of the 

 embryo extends out in a number of finger-like processes into 

 the trophoblast layer, and soon afterwards blood vessels 

 shoot into these, and the ehorionie villi are formed (fig. 176). 

 The precise origin of the first blood vessels in these 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 in the maternal 

 mucosa (fig. 175, D.S.) dilate, and the capillaries form large 

 sinuses or blood spaces. Into these the ehorionie 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. This development goes on specially at one 



