196 JOHN LEWIS BREMER 



tion of the uterine glands and of the surface epithehum, with 

 many maternal leucocytes added. Respiration and nutrition 

 must be active secretory processes, as nowhere in these placentae 

 is found any thin osmotic membrane in relation with the fetal 

 capillaries. Granules of absorbed material have often been 

 found in the fetal epithelium. 



The conjugate placenta, on the other hand, is formed by a 

 destruction of the maternal tissue by the trophoderm, a loss of 

 the uterine surface epithelium and of more or less of the 

 deeper layers, and the consequent pouring out of the maternal 

 blood into the intervillous spaces. The chorion is bathed in 

 slowly circulating maternal blood, and it is from this, instead 

 of from the uterine milk, or by the transference from one epithelial 

 cell to another, that the embryo obtains food material and oxy- 

 gen. The maternal epithelium of the uterus plays no part; 

 the seat of the transfer is the epithelial covering of the chorion 

 and its prolongations, the chorionic villi. 



An exception seems to exist in those rodents with the so-called 

 'inversion of the germ layers,' for in these the yolk-sac entoderm, 

 after the loss of its distal layers and of the chorion originally 

 covering it, is spread out over a portion of the inner surface of 

 the uterus, and may receive nutriment from the secretion of 

 the uterine glands. But this is at best an accessory source, as 

 the placenta proper is in these cases also composed of chorionic 

 prolongations, bathed in circulating maternal blood. Another 

 exception is found in the 'green column' or border zone of the 

 zonate placenta of the cat and other carnivora, a large reser- 

 voir of extravasated blood, between the uterus and the chorion, 

 from which the chorionic epithelial cells, here of a tall cylindrical 

 type, can be seen to ingest certain red blood corpuscles. This 

 again is an accessory source of food supply, as it develops onl}^ 

 in the second half of pregnancy, and has no noticeable effect on 

 the size or activity of the true placenta. 



It is the characteristic of conjugate placentae, then, that 

 the fetal chorion and ^^illi, of whatever shape they may be, are 

 bathed in circulating maternal blood, and that the transference 

 of material between mother and embryo takes place through the 



