MAiMMALIA. 209 



ticula allantoidis. The cavity of the allantois remains large, but the 

 lining of h3'pob1ast becomes separated from the mesoblast, owing to the 

 more rapid growth of the latter. The mesoblast of the allantois applies 

 itself externally to the subzonal membrane to form the chorion', and in- 

 ternally to the amnion, the cavity of which remains very small. The 

 cliorionic portion of the allantoic mesoblast is very vascular, and that 

 applied to the amnion also becomes vascular in the later developmental 

 periods. 



The horns of the yolk-sack gradually atrophy, and the whole yolk- 

 sack disappears some time before birth. 



Where two or more embryos are present in the uterus, the chorions of 

 the several embryos may unite where they are in contact. 



From the chorion there grow out numerous vascular villi, which fit into 

 corresponding pits in the uterine walls. According to the distribution of 

 these villi, the allantois is either diffused or polycotyledonary. 



The pig presents the simplest type of diffused placenta. The villi of 

 the surface of the chorion cover a broad zone, leaving only the two poles 

 free ; their arrangement differs therefore from that in a zonary placenta 

 in the greater breadth of the zone covered by them. The villi have the 

 form of simple pajnllre, arranged on a series of ridges, which are highly 

 vascular as compared with the intervening valleys. If an injected chorion is 

 examined (fig. 154\ certain clear non-vascular spots are to be seen (h), from 



Fig. 154. Pokxiun of tuk injected chokion of a Pio, slightly magnified. 



(From Turner.) 

 The figure shews a minute circular spot {h) (enclosed by a vascular ring) from 

 which villous ridges (r) radiate. 



which the ridges of villi radiate. The surface of the uterus adapts 

 itself exactly to the elevations of the chorion ; and the furrows which 

 I'eceive the chorionic ridges are highly vascular (fig. 155). On the other 

 hand, there are non-vascular circular depressions corresponding to the non- 

 vascular areas on the chorion ; and in these areas, and in these alone, the 



1 According to Bisclioff the subzonal membrane atrophies, leaving the allantoic 

 mesoblast to constitute the whole chorion. 



B. K. II. 14 



