FCETAL NUTRITION: THE PLACENTA 427 



in. The Placenta ix IxnECiDUATA 



In the placental JMammals an attaclnnent takes place between 

 maternal and fcetal tissues in the uterus, and tlie troplioblast is 

 vascniarised, except in tlie Primates, by the allantois. The method 

 of attachment varies in diffeient oideis, and sometimes in different 

 groups of an (irder. In the Indeciduata, however, the first attach- 

 ment is always obtained liy an Kppimtinn of tlie troplioblast to the 

 sni'face of the nnicosa. 



Ungulata: Piij. — In the pig tlie lilastocysts are spherical till the 



Fig. 112. — Portion of the injected chorion of the pig. The ligure shows a 

 minute circular spot [h), enclosed by a vascular ring, from which villous 

 i-idges (/■, /•) radiate (Turnei'). (From Balfour's Compriraticf EinhniiAogi/, 

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



tenth day. Then they raj)idly elongate, and by tlie 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 i), which fit lietween ridges of the uterine mucosa. The 

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

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

 the gland-mouths (Turner-). It is usuall)' stated that tlie uterine 

 surface epitlielium remains intact ; but Assheton has recently proved 

 that it shows signs of degeneration as eaily as the eighth (\a\, 

 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 troplioblast fits closely, 



' Assheton, Phil. Trfinn,, lor. rit. 



-Turner, Lcrturcs on the CoinpamUve Anatomi/ of th<> Phtrento, Edinburgh, 

 1876. 



