DEVELOPMENT. 



783 



processes, the so-called chorion villi (a, figs. 478, 479), which give it 

 a rough and shaggy appearance. At first only cellular in structure, these 

 little outgrpwths subsequently become vascular by the development in 





Fig. 478. 



Fig. 479. 



Figs. 478 and 479. 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 

 placenta ; 6, space between the two layers of the amnion ; c, amniotic cavity ; d, situation of the 

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

 heart and vessels ; <;, allantois. 



them of loops of capillaries (fig. 480) ; 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, 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 devel- 

 oped, 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 impreg- 

 nation. These changes consist especially of alterations in structure of 

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

 the uterus, and which forms, after a kind of development to be imme- 



Fig. 480. 



