394 THE PHYSIOLOGY OF REPRODUCTION 



III. THE PLACENTA IN INDECIDUATA 



In the placental Mammals, an attachment takes place be- 

 tween maternal and foetal tissues in the uterus, and the tropho- 

 blast is vascularised, except in the Primates, by the allantois. 

 The method of attachment varies in different orders, and 

 sometimes in different groups of an order. In the Indeciduata, 



FIG. 87. Portion of the injected chorion of the pig. The figure shows a 

 minute circular spot, 6, enclosed by a vascular ring from which villous 

 ridges (r, r) radiate (Turner). (From Balfour's Comparative Embryology, 

 vol. ii. By permission of Messrs. Macmillan & Co., Ltd.) 



however, the first attachment is always obtained by an apposi- 

 tion of the trophoblast to the surface of the mucosa. 



UNGULATA : Pig. In the pig the blastocysts are spherical till 

 the tenth day. Then they rapidly elongate, and by the four- 

 teenth day they fill the whole length of the uterus. Subsequently 

 they obtain a greater surface of contact by a series of concer- 

 tina-like foldings (Assheton 1 ), which fit between ridges of the 

 uterine mucosa. The ridges are inter-glandular in position 



1 Assheton, Phil. Trans., loc. cit. 



