216 EVOLUTION OF THE PLACENTA. 



mesh work, and in the interior of the trabeculse there run dilated maternal 

 capillaries {d'). The trabeculse are covered by a more or less columnar 

 uterine epithelium (e), and are in contact on every side with fcetal villi. 

 The capillaries of the fcetal villi preserve their normal size, and the villi 

 are covered by a flat epithelial layer (e). 



In the sloth (F) the maternal capillaries become still more dilated, and 

 the epithelium covering them is formed of very flat polygonal cells. 



In the human placenta (G), as in that of Apes, the greatest modi- 

 fication is found in that the maternal vessels have completely lost their 

 capillary form, and have become expanded into large freely commu- 

 nicating sinuses (c^). In these sinuses the fcetal villi hang for the most 

 part freely, though occasionally attached to their walls {t). In the late 

 stages of foetal life there is only one epithelial layer (e') between the 

 maternal and foefcal vessels, which closely invests the fcetal villi, but, as 

 shewn by Turner and Ercolani, is part of the uterine tissue. In the foetal 

 villi the vessels retain their capillary form. 



Evolution of the Placenta. 



From Owen's observations on the Marsupials it is clear that the 

 yolk-sack in this group plays an important, if not the most important 

 part, in absorbing the maternal nutriment destined for the foetus. 

 The fact that in Marsupials both the yolk-sack and the allantois are 

 functional in rendering the chorion vascular makes it a priori 

 probable that this was also the case in the primitive types of the 

 Placentalia, and this deduction is supported by the fact that in the 

 Rodentia, Insectivora and Cheiroptera this peculiarity of the foetal 

 membranes is actuall}^ found. In the primitive Placentalia there 

 w^as probably present a discoidal allantoic region oi the chorion, from 

 which simple foetal villi, like those of the pig (fig. 161 B), projected 

 into uterine crypts ; but it is not certain how far the umbilical part 

 of the chorion, which was no doubt vascular, may also have been 

 villous. From such a primitive type of foetal membranes divergences 

 in various directions have given rise to the types of foetal membranes 

 now existing. 



In a general way it may be laid down that variations in any 

 direction which tended to increase the absorbing capacities of the 

 chorion would be advantageous. There are two obvious ways in 

 which this might be done, viz. (1) by increasing the complexity of 

 the foetal villi and maternal crypts over a limited area, (2) by in- 

 creasing the area of the part of the chorion covered by placental villi. 

 Various combinations of the two processes would also of course be 

 advantageous. 



The most fundamental change which has taken place in all the 

 existing Placentalia is the exclusion of the umbilical vesicle from 

 any important function in the nutrition of the foetus. 



The arrangement of the foetal parts in the Rodentia, Insectivora 

 and Cheiroptera may be directly derived from the primitive form by 

 supposing the villi of the discoidal placental area to have become 

 more complex, so as to form a deciduate discoidal placenta; while the 



