CHANGES LV THE OVUM, 



73 



lined by the external layer of the blastoderm. It is thought that the 

 primitive chorion disappears by resorption, and that this blastodermic 

 layer becomes the definitive chorion ; also that when the allantois has 

 spread itself between the amnion and the external envelope, carrying 

 outward the umbilical vessels, this definitive chorion becomes vascular, 

 and furnished with its numerous villosities. 



Differences. / 



Ru7ninaiits. 



In Ruminants, the chorion corresponds to the internal face of the ute- 

 rus, whose form it repeats, and with w^iose surface it is more or less in 

 contact. The middle of its inner surface is united to the amnion and the 

 allantois by loose gelatinous connective tissue, so laminated that it might 

 be mistaken for different layers of membrane. It is only covered by the 

 allantois in the portions corresponding to the cornua, which are longer 

 than those of the uterus. It offers numerous small red masses, studded 

 at variable distances from each other on the surface next the uterus, and 

 which effect a very important connection between the latter and the cho- 

 rion. These are the placentulce to which we shall allude presently. 



In the Sheep and Goat, when there are two young creatures, the two 

 cornua of the chorion are joined, so as to look externally like one sac. 



Fig. 3?- 

 Foetus and Fcetal Membranes of the Cat. 

 a^. Chorion ; b, Zonular Placenta ; d, Umbilical Vesicle, with its Expansions, y, between 

 Amnion and Placenta, and ^, its Pedicle of attachn.ent to a loop of small intestine ; k, 

 Allantois ; k, Fcetus. 



Pig. 



In the Pig there are no cornua, but the whole appears as an elongated 

 sac, whose two extremities, much exceeding those of the embryos, are in 

 relation with the envelopes of the contained progeny. Its internal face 



