CHANGES IN THE OVUM. 8i 



crypts in the uterine mucous membrane, bring the capillary systems of 

 mother and foetus into the closest relationship : only the very thin coats 

 of the vessels and the epithelium intervening in the two circulations. 

 There is no fusion, vascular continuity, or direct communication be- 

 tween the maternal and foetal systems, as was at one time taught \ all 

 the important changes that occur taking place through the walls of the 

 capillaries by virtue of osmotic force. 



The function of the placenta, then, is to administer to the nutrition 

 and development of the foetus by means of its intimate relations with 

 the uterine vascular system, until the time has arrived for the expulsion 

 of the young animal. Notwithstanding their close apposition, the ad- 

 herence of this papillary layer of the chorion with the inner surface of 

 the uterus is so slight, that this organ can scarcely be opened w^ithout 

 more or less destroying it. A small quantity of a brownish fluid is found 

 between the two. 



At an early stage of gestation there is no placenta \ a temporary mass 

 of albuminoid substance accumulating around the ovum in the uterus, 

 affords material for its nourishment until the vascular intussceptive re- 

 lations between the chorion and uterine lining is established. When 

 gestation is terminated, the placenta becomes remarkably rigid, the ves- 

 sels are obliterated and transformed into fibrous tissue, and the external 

 face of the chorion is wrinkled and withered-looking. 



I 



Fig. 41. 



Portion of Chorion with Placentul.*: : Cow. 



1, Chorion ; 2, Placentulse. 



DiFFERENOeS. 



Cow. 



In Ruminants, there is an important difference m the arrangement of 

 the placenta from that just described. In the Cow, the villi of the 

 chorion are developed and agglomerated in large numbers at certain 

 points of its surface, to constitute a multiple or tufted placenta, which is 

 composed in this way of from sixty to oXghty placentulcz, or "foetal coty- 

 ledons." These are of a bright red color, of various sizes, and gen- 

 erally oval in shape ; they correspond to the prominences on the lining 



6 



