752 GENERATION AND DEVELOPMENT. 



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 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 foetal 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 impregnation. 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 immediately described, the meinbrana 

 decidua, so called on account of its being discharged from 

 the uterus at the period of parturition. 



Changes of the Mucow Membrane of the Uterus, and Formation 

 of the Placenta. 



The mucous membrane of the human uterus is abun- 

 dantly beset with tubular follicles, arranged perpen- 

 dicularly to the surface. These follicles are very small in 

 the unimpregnated uterus; but when examined shortly 

 after impregnation, they are found elongated, enlarged, 

 and much waved and contorted towards their deep and 

 closed extremity, which is implanted at some depth in the 

 tissue of the uterus, and commonly dilates into two or 

 three closed sacculi (fig. 226). 



