CHANGES IN THE OVUM. 85 



-delicate. The mode of termination of the placental vessels in the Sheep 

 is villous ; in the Goat it is pilose. 



Pig. 



In the Pig the placenta maybe designated as "diffused," not " polycot- 

 yledonary " as in Ruminants; though the tufts do not form a continuous 

 layer as in the Mare, the papillae being collected in small but closely- 

 grouped clusters, which give the chorion a mottled aspect. When unin- 

 jected they appear as white masses or spots scattered over the external 

 surface of the chorion, thus giving it a mottled aspect, caused by the 

 presence of feebly vascular and non-vascular areas ; but when the allantoic 

 veins are filled, these are seen to form plexuses in the centre of each spot. 

 The uterine veins have a corresponding disposition, and the arterial 

 capillaries form a fine network, the meshes receiving the villosities which 

 carry the fcetal arterial capillaries ; whence it might seem that the nutri- 

 tion of the fcetus was effected principally at the points of contact of the 

 foetal with the maternal venules, while the respiratory process took place 

 at the surface of contact between the foetal and maternal arterial capil- 

 laries. The chorion is destitute of these villosities at the extreniities, 

 which are in contact with the chorion of others foetuses in the uterus. 



Bitch and Cat. 



In the Bitch and Cat the placenta forms a thick annular band or zone, 

 about one or one and a half inch wide, passing round the middle of the 

 chorion ; it is therefore said to be " zonular." This zone is concave 

 within, of a mixed grey-and-red color when uninjected, livid or dark 

 brown during gestation ; its foetal surface is lobulated, and the zone is 

 limited at each side by a dark green border, the coloring matter of which 

 can scarcely be removed by repeated washings. 



The placenta is studded with ramified villi of a leaf or plate shape, 

 which are implanted in the uterine mucous follicles. The mucous mem- 

 brane in which these are situated, and which corresponds to the placenta, 

 presents a kind of vascular fungus development that appears after 

 parturition, but which at an early stage of pregnancy has a quantity of 

 fluid along its margin; the maternal placenta, ovserotine decidua, is 

 present during gestation, and can be separated as a distinct layer. 

 There cannot be a doubt that the uterine mucous membrane in car- 

 nivora secretes a kind of plastic lymph, which forms this caducous 

 lining or membrana decidua ; but it is only present at a certain period 

 of foetal life, and forms the base of the uterine placenta. 



Functions. 



The functions of the placenta are obvious: it is the nutrient and 

 respiratory apparatus during a portion of intra-uterine existence ; and for 

 the accomplishment of these functions it must rely upon its intimate and 

 healthy relations with the uterine surface. The special and temporary 

 processes of development being completed and the task of providing 

 capillary superficies being terminated, whether on the part of the mother 

 or foetus, the placenta of the latter disappears, as well as the decidua ; 

 though they may not be thrown off together, and the maternal decidua 

 may not be shed all at once, but in successive portions. Th# long period 

 of gestation necessary to endue the young of defenceless hoofed animals 

 with sufficient strength before birfh, is perhaps a reason for the firmer 



