496 PROFESSOR TURNER ON THE GRAVID UTERUS AND 



or zone-like form of placenta. It is quite unnecessary for me to go into the 

 proofs on which this statement rests, as ample evidence of its accuracy has been 

 already advanced by Professors Eschricht and Rolleston in their valuable 

 memoirs already so frequently referred to. The shedding of the placental portion 

 of the serotina at the time of parturition, has led zoologists to class all those 

 mammals together in which it occurs as caducous or deciduate mammals. 



We may now inquire if there are any structures in the Orca, and inferen- 

 tially in the other mammals which possess a "diffused" placenta, at all 

 comparable to the decidua serotina, although it may not be actually shed along 

 with the foetal membranes. 



I have already stated that the cells of the serotina intervene between the 

 villi and the maternal blood-vessels. In the Orca we have also seen that 

 although the vessels of the uterine mucous surface are not dilated into sinuses, 

 but preserve the form of ordinary capillaries, yet that they are separated from 

 the epithelial investment of the villi, not only by the epithelial cells lining the 

 crypts, but by the sub-epithelial corpuscles of the mucous membrane. In their 

 anatomical position these layers of cells correspond therefore to the cells of the 

 serotina. We have no evidence at present that these layers are separated at 

 the time when the membranes come away, though I think it very probable if 

 the chorion of a whale or of a mare, investing a foetus born at the full period of 

 gestation, were carefully examined, that the epithelial lining of the crypts might 

 to some extent at least be found to be shed along with it. But during the period 

 of involution which follows parturition, it is obvious that great changes, either 

 from actual shedding of portions of its substance, or from degeneration and 

 interstitial absorption, must take place in these and the other constituents of 

 the crypt layer before it . can be restored to its proper non-gravid condition. 

 The difference between the diffused placenta of a whale and the concentrated 

 placenta in the human subject appears therefore to be not an essential difference 

 in the presence or absence of certain anatomical structures, but rather a 

 difference in arrangement. In the whale the cell structures developed in con- 

 nection with the uterine mucous membrane, and which occupy the position of 

 the decidua serotina, are diffused over an extensive surface, and possess a simple 

 laminated arrangement, and the maternal blood-vessels do not lose their capillary 

 characters. The chorionic villi, also, are lodged in comparatively shallow crypts, 

 out of which they can be easily enucleated, either with or without the simul- 

 taneous shedding of the cell-layers of the crypts. In the human subject, on the 

 other hand, the cell structures of the serotina, developed in connection with the 

 uterine mucous membrane, are concentrated in a limited area, and so inter- 

 mingled with the chorionic villi, that, when these are separated, the cells of the 

 decidua must necessarily be shed at the same time. The maternal blood- 

 vessels also assume the form and size of sinuses. Further, I believe that these 



