THE HUMAN PLACENTA. 101 



terior lobes, while, in the three other genera of anthropoid apes, 

 the posterior lobes of the cerebrum project behind the cere- 

 bellum.* 



The latest form of the " cerebral " classification of the Mam- 

 malia having thus been shown to be devoid of any sound 

 foundation, I proceed to inquire whether the " placental' 

 classification does, or does not, stand upon a more secure basis, 

 if we take, not merely, with Milne-Edwards, the form of the 

 placenta, but with Von Baer and Eschricht, its structure, into 

 account. It is a well-established fact that two very distinct 

 types of placenta are to be met with in the Monodetyhia, and 

 that, at the present moment, we have no knowledge of any 

 transitional forms between these two types. The first of these 

 types is that exhibited by the human placenta, the second by 

 that of the pig or horse. 



From the commencement of gestation, the superficial sub- 

 stance of the mucous membrane of the human uterus undergoes 

 a rapid growth and textural modification, becoming converted 

 into the so-called " decidua" While the ovum is yet small, 

 this decidua is separable into three portions, — the decidua vera, 

 which lines the general cavity of the uterus ; the decidua 

 reflexa, which immediately invests the ovum ; and the decidua 

 serotina, a layer of especial thickness, developed in contiguity 

 with those chorionic villi which persist and become converted 

 into the foetal placenta. The decidua reflexa may be regarded 

 as an outgrowth of the decidua vera ; the decidua serotina as 

 a special development of a part of the decidua vera. At first, 

 the villi of the chorion are loosely implanted into correspond- 

 ing depressions of the decidua ; but, eventually, the chorionic 

 part of the placenta becomes closely united with, and bound to, 

 the uterine decidua, so that the foetal and maternal structures 

 form one inseparable mass. 



In the meanwhile, the deeper substance of the uterine 



* See Mr. Flower's paper " On the Brain of the Siamang," Natural History 

 Review, April, 1863. " This peculiarity of the Siamang's brain is due to two 

 causes — firstly, the large development of the cerebellum ; secondly, and I shall 

 afterwards show, mainly, to the actual shortness of the posterior or occipital lobe 

 of the cerebrum." — L. c, p. 282. 



