STRUCTURE OF HUMAN PLACENTA. 451 



stems, are the germinal spots of the villi. They are the active 

 agents in the formation of these parts. The villus elongates 

 by the addition of cells to its extremity, the cells passing off 

 from the germinal spot, and the spot receding on the extremity 

 of the villus, as the latter elongates by the additions which 

 it receives from it. 



The bulbous extremities of the villi of the chorion, are not 

 only the formative agents of these parts, but are also all along, 

 but principally after the villi have become well developed, 

 their functional agents also. They are to the ovum what the 

 spongioles 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 bloodvessels. Bloodvessels 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 villi, and 

 become continuous with loops in their interior. Those villi in 

 which the bloodvessels do not undergo any further develop- 

 ment, 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 in 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 bloodvessels 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 membrane, 

 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 

 coalesces with the contained cells. 



