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ANATOMY -OF VERTEBRATES. 



out of the accumulated decidual substance (15th day): after 

 fulfilling that office the allantois disappears, or is represented by 

 the delicate sheath inclosing the trunks of the allantoic arteries 

 and veins, ib. d. The amnios, a, in the meanwhile, has been 

 completed : and the embryo, so inclosed, is suspended freely 

 within the cavity of the vitellicle, which, by disappearance of the 

 primitive vegetal layer, is now directly surrounded by a deposit 

 of decidua, fig. 573, /, becoming thinner as the foetus and its 

 vitellicular membrane expands ( 17th day). Through this order of 

 development it seems that the relative position of the embryo to its 

 appendages is the reverse of that in the Rabbit, fig. 571. The 

 mesometral mass of decidua forms a well-defined thick circular 

 placenta, ib. f, lobulated on the inner free surface by furrows 

 affecting a radiate but anastomosing disposition, to the centre of 



which pass the allantoic ves- 

 sels. These, however, began 

 on reaching the decidua, to 

 organise a distinct placental 

 mass, ib. e, which might be 

 termed the foetal portion : it, 

 however, receives maternal 

 vessels from the larger and first 

 formed decidual placenta, ib. 

 f. The proportions of these 

 placentae become reversed as 

 the foetus grows. The uterine 

 veins, before quitting the pla- 

 centa, form an annular sinus 

 around the portion e, and then 

 penetrate the decidual parts,/, g, and the uterus. The allantoic 

 vessels also ramify not only in e, but also in f, between the lo- 

 bules of which the larger branches pass to gain the periphery. 



As the foetus approaches its term the decidual covering of the 

 ovum disappears, or is reduced to mere shreds at the circum- 

 ference of the placental enlargement,/*, which is now much re- 

 duced in size : it receives into a cavity a mammilloid process 

 from the centre of the foetal placenta, to which process converge 

 the uterine vessels from without and the foetal vessels from 

 within, before they ramify to the periphery. The vitellicle still 

 represents the chorion : its arteries form a peripheral l circulus 

 arteriosus,' parallel with the vena terminalis or ( circulus venosus,' 

 and in the interspace of these vascular circles, fimbriate processes 

 grow out richly supplied by vitelline vessels and constituting a 



Diagrammatic section of embryo of Guinea-pig 1 , with 

 its vitelline and decidual membranes and placen- 

 tas. CCLXII". 



