DEVELOPMENT. 673 



itself become previously fused with the vitelline membrane. By these 

 means the external investing membrane of the ovum, or the true chorion, 

 as it is now called, represents three layers, namely, the original vitelline 

 membrane, the outer layer of the amniotic fold, and the allantois. 



Very soon after the entrance of the ovum into the uterus, in the 

 human subject, the outer surface of the chorion is found beset with 

 fine processes, the so-called villi of the chorion (a, Figs. 458, 459), 

 which give it a rough and shaggy appearance. At first only cellular 

 in structure, these little outgrowths subsequently become vascular by 

 the development in them of loops of capillaries (Fig. 460); and the latter 

 at length form the minute extremities of the blood-vessels which are, so 

 to speak, conducted from the foetus to the chorion by the allantois. 

 The function of the villi of the chorion is evidently the absorption of 

 nutrient matter for the foetus; and this is probably supplied to them at 

 first from the fluid matter, secreted by the follicular glands of the uterus, 



FIGS. 458 and 459. a, chorion with villi The villi are shown to be best developed in the 

 part of the chorion to which the allantois is extending; this portion ultimately becomes the pla- 

 centa; 6, space between the two layers of the amnion; c, amniotic cavity; d, situation of the intes- 

 tine, showing its connection with the umbilical vesicle; e, umbilical vesicle;/, situation of heart 

 and vessels; gr, allantois. 



in which they are soaked. Soon, however, the foetal vessels of the villi 

 come into more intimate relation with the vessels of the uterus. The 

 part at which this relation between the vessels of the foetus and those of 

 the parent ensues, is not, however, over the whole surface of the chorion: 

 for, although all the villi become vascular, yet they become indistinct or 

 disappear except at one part where they are greatly developed, and by 

 their branching give rise, with the vessels of the uterus., to the formation 

 of the placenta. 



To understand the manner in which the fatal and maternal blood- 

 vessels come into relation with each other in the placenta, it is necessary 

 briefly to notice the changes which the uterus undergoes after impregna- 

 tion. These changes consist especially of alterations in structure of the 

 superficial part of the mucous membrane which lines the interior of the 

 43 



