FCETAL NUTRITION: THE PLACENTA 427 



III. THE PLACENTA IN INDECIDUATA 



In the placental Mammals an. attachment takes place between 

 maternal and foetal tissues in the uterus, and the trophoblast 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, however, the first attach- 

 ment is always obtained by an apposition of the trophoblast to the 

 surface of the mucosa. 



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



FIG. 112. Portion of the injected chorion of the pig. The figure shows a 

 minute circular spot (ft), enclosed by a vascular ring, from which villous 

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

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



tenth day. Then they rapidly elongate, and by the fourteenth day 

 they fill the whole length of the uterus. Subsequently they obtain a 

 greater surface of contact by a series of concertina-like foldings 

 (Assheton : ), which fit between ridges of the uterine mucosa. The 

 ridges are inter-glandular in position (Fig. 112), radiating from small 

 circular spots, twenty or thirty to the square inch, which represent 

 the gland-mouths (Turner 2 ). It is usually stated that the uterine 

 surface epithelium remains intact ; but Assheton has recently proved 

 that it shows signs of degeneration as early as the eighth day, 

 and at the eighteenth day is reduced to a thin layer. Three days 

 later, however, it again appears normal and is formed of long 

 columnar cells, to the ends of which the trophoblast fits closely, 



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



2 Turner, Lectures on the Comparative Anatomy of the Placenta, Edinburgh, 

 1876. 



