1024 EMBRYOLOGY. 



nucleated cell, the protoplasm in which is intersected by vessels — is of opinion 

 that the placental cavities in the Mare are of the same character, except that 

 there are several giant cells. There is nothing glandular in the epithelium 

 lining these cavities ; on the contrary, they are often confounded with the cells 

 of the deeper part. 



These cavities are, then, sinuses lined with blood-vessels, and are not glands. 

 Besides, as may be seen in examining Fig. 559, they are quite distinct from the 

 proper tubular glands of the uterine mucous membrane. 



Structure and Development. — The villi of the placenta have for their 

 base a tissue similar to that of the chorion, their stroma being formed by bundles 

 of connective tissue mixed with cells. They are covered by a simple cylindrical 

 epithelium placed immediately on the connective stroma (Fig. 56oi Their 



Fig. 559. 



VERTICAL SECTION OF THE INJECTED PLACENTA OF A MARE. 



ck, Chorion, with its villi partly m situ, partly torn out of the uterine sinuses, cr ; g, uterine 

 glands ; V, blood-vessels ; E, epithelium. 



interior is occupied by a capillary network, the finest divisions of which are 

 subjacent to the epithelium ; the network itself is supplied by a branch from the 

 umbilical arteries, and the blood is returned by a vein nearly in the centre of 

 the villus. 



The vessels pre-exist in the villi ; consequently, the latter do not develop 

 from the chorial tissue, but rather, it might be said, the chorion is pushed back 

 by the pressure of the vessels-. The placenta succeeds, then, the development of 

 the allantois, the use of which is to convey the umbilical vessels from the 

 umbilicus to the chorion. 



The villi penetrate the uterine mucous membrane, in such a way that the 

 capillary systems of the mother and foetus are only separated by the thin walls 

 of the vessels, and the epithelium of the villi and follicles. 



The fusion of these two systems has never been observed, and all the inter- 

 changes between the female and its young take place through the capillaries by 

 osmotic force only. 



