198 JOHN LEWIS BREMER 



In the guinea pig, whose gestation is twice as long as that of 

 the rabbit, it appears at about the same time, that is relatively 

 early, the embryo in the same period having attained little more 

 than half the size of the rabbit embryo. The modification of 

 the trophoderm is still more precocious in the rat, for at 13 

 days in the rat the succession of plates and cell bodies is easily 

 recognizable (fig. 6). 



By referring to the figures given in former paragraphs (page 

 187), one can readily see that the dates of the development of 

 the plates in the placenta correspond accurately with those of 

 the involution of the Wolffian body, seen in the rabbit at about 

 21.0 mm. and in the guinea pig quite advanced at 15.0 mm. 

 In the rat, since no glomeruli ever develop, the placental plates 

 are found at about the period in which the mesonephric glomeruli 

 become active in other mammals. The development of possible 

 osmotic membranes in the placentae of various rodents at the 

 precise time when such membranes are lacking in the cor- 

 responding embryos can hardly, it seems to me be a mere 

 coincidence. 



Another type of conjoined placenta is that found in most car- 

 nivora, of which the cat and the dog have usually been taken as 

 types. A placental labyrinth is present, with what corresponds 

 to the chorionic villi; the separate villi are not free, however, 

 but are bound together by their trophoderm, through which the 

 maternal blood has made a network of channels. Everywhere 

 between the maternal blood stream and the fetal capillaries in 

 the mesoderm of the villi the thick syncytial layer of the tropho- 

 derm intervenes. Nowhere does this layer become changed into 

 membranous plates. At each edge of the zonate placenta there 

 develops at about the middle of pregnancy the 'green column' 

 of which mention has been made. Surprised at first by the \'ery 

 small excreting surface exhibited by the glomeruli of the Wolf- 

 fian body and kidney of the cat, I sought in this 'green column' 

 for a possible osmosis between fetal and maternal blood, but 

 only to find that there is here no maternal circulation, merel}' 

 an extravasation, and that no sign of a plate-like chorionic epi- 

 thelium exists amid the peculiarly high columnar cells. In the 



