J018 ~^ EMBRYOLOGY. 



to the envelopes sniToundiiig it, and which ramify in the placental tufts (Fig. 

 556). 



1. The Chorion (Figs. 554, 555). 



The outermost envelope of the ovum, the chorion is a vast membranous and 

 perfectly closed sac, the shape of which exactly corresponds with that of the uterus. 

 It has a hodij and two cornua. The latter, when inflated, are plicated and 

 sacculated like the caecum, and are always unequal in volume — that in which 

 the foetus is developed having by far the largest dimensions.^ 



The external face is studded with small red tubercles, formed by the placental 

 tufts. It adheres to the internal surface of the uterus. Between the two 

 membranes there is found a small quantity of sanguinolent fluid ; but on one 

 occasion, in the Ass, it had the appearance of milk. 



The internal face, lined by the external layer of the allantois, is united in 

 the closest manner to that membrane, except at the umbilical cord, where there 



Fie. 555 



EXTERIOR OF THE CHORIAL SAC (MARE). 



A, Body ; B, C, cornua. 



exists a kind of conical infundibulum occupied by the umbilical vesicle. Ad- 

 hesion is established by means of the vessels of the umbilical cord passing into 

 the texture of the chorion, and by very delicate, laminal, interannexial connective 

 tissue. 



Structure and Development. — The external covering of the ovum is at 

 first formed by the vitelline tunic, itself constituted by the transparent membrane, 

 covered externally by a layer of cells derived from the Fallopian tube or uterus. 

 This primitive chorion is replaced by a portion of the extra-foetal somatopleure, 

 which becomes the secondary or definitive chorion. It is destitute of vessels until 

 the allantois is developed. 



When the chorion is complete, it is composed of three layers : 1. An external 

 epithelial covering, formed of a single layer of uniform cylindrical cells, contain- 



' When there is only one foetus, it is always more particularly developed in one comu, 

 which consequently has an enormous volume when compared with the other. When there 

 are two foetuses— as often happens in the Ewe — there is one in each cornu; and then the 

 cornua develop in the same manner. 



