56 THE STRUCTURE OF THE HUMAN PLACENTA. 



but principally after the villi have become well developed, their 

 functional agents also. They are to the ovum what the spon- 

 geoles are to the plant they supply it with nourishment from 

 the soil in which it is planted. 



Up to a certain period of gestation, the chorion and its villi 

 contain no blood-vessels. Blood-vessels first appear in these 

 parts when the allantois reaches and applies itself to a certain 

 portion of the internal surface of the chorion. The umbilical 

 vessels then communicate with the substance of the viUi, and be- 

 come continuous with loops in their interior. Those villi in which 

 the blood-vessels do not undergo any further developement, as 

 the ovum increases in size, become more widely separated, and 

 lose their importance in the economy. The villi, again, in which 

 vessels form, in connection with the umbilical vessels, increase in 

 number, and undergo certain changes ki the arrangement of their 

 constituent elements, so as to become the internal structures of 

 the tufts of the placenta, as described in the first part of this 

 Memoir. The villi of the chorion always retain their cellular 

 structure. As the blood-vessels increase in size the cells diminish 

 in number ; but are always found surrounding the terminal loop 

 of vessels in the situation of the germinal spot. The fine mem- 

 brane, which was formerly described as bounding the villus of 

 the chorion, always remains at the free extremities of the villi of 

 the placenta ; but on the stems and branches of the latter it coa- 

 lesces with the contained cells. 



The conversion into fibrous texture of the membrane and cells 

 of the stems and branches of the tuft of the chorion, forms the 

 tough white fibrous trunk and branches of the tufts of the foetal 

 portion of the placenta ; in each of which runs a branch of the 

 umbilical arteries and vein ; and the fine membrane of the villi 

 of the .chorion, with its contained cells and terminal blood-loops, 

 still persistant at the extremities of the villi, are the internal 

 membrane, the internal cells, and the blood-loops described in 

 the first part of this memoir, 



III. OF THE MATERNAL PORTION OF THE PLACENTA. 



The mucous membrane of the uterus presents on its free sur- 



