614 THE HUMAN EMBRYO. 



placenta the foetal and maternal blood streams are kept apart^ 

 and tliat no actual mixing of- blood from the two sources can 

 occur. 



The real nature of the sinuses, or loculi, in which the maternal 

 blood lies has been much discussed, and is not yet determined 

 with certainty. 



It was formerly held that they are produced by enormous 

 dilatation of the capillaries, which normally connect the uterine 

 arteries and veins together ; and Waldeyer has shown that these 

 sinuses have a distinct epithelial lining, continuous with that of 

 the uterine vessels. 



Kolliker and Langhans point out that, on the fcetal side of the 

 placenta, the walls of the sinuses are formed by the chorion, and 

 show no trace of decidual structure ; they, therefore, suggest that 

 the sinuses are not really maternal capillaries, but are .spaces 

 between the maternal and foetal portions of the placenta, i.e. 

 between the decidua and the chorion, into which the blood has 

 penetrated by extravasation, or by rupture of the uterine vessels, 

 consequent on the degeneration which the uterine mucous 

 membrane is known to undergo. 



On the other hand, a comparison of what is known concerning 

 the human uterus, with the facts ascertained in regard to the 

 formation of the placenta in the rabbit and in other Mammals, 

 leads to the belief that the sinuses may prove to be spaces formed 

 by absorption, not within the maternal tissues, but in the 

 chorionic epithelium itself; in which case the whole thickness 

 of the villous zone of the placenta would be of foetal origin. 



Separation of the Placenta at Birth. 



In parturition, the contraction of the muscular walls of the 

 uterus, and the consequent pressure on the uterine contents, 

 more especially upon the amnionic fluid, causes the investing 

 membranes of the embryo, i.e. the combined decidua vera and 

 decidua reflexa, the chorion, and the amnion, to bulge through 

 the OS uteri. On rupture of the membranes, the amnionic fluid 

 first escapes, and subsequently the foetus is expelled. 



The further contraction of the uterus detaches the placenta 

 from the uterine wall, the plane of separation passing through 

 the outer or spongy layer of the decidua, in which the deeper 

 parts of the uterine glands still persist ; and the placenta, with 



