VILLI OF THE PLACENTA. 495 



These sinuses, which traverse the whole placenta, and project 

 freely into the foetal villi, are exclusively bounded by the 

 placentary tissues. Kolliker and Bidder * were unable to dis- 

 cover the presence of the thin membrane described by E. H. 

 Weberf as forming an investment to the maternal cavities. 



The foetal portion the placenta foetalis is formed by the 

 development of the villi of the chorion, in which are distributed 

 branches of the two arteries and of the vena umbilicalis. 



The villi of the placenta foetalis have very recently been 

 subjected to renewed examination by Jassinsky. He corro- 

 borates the statement that the villi are invested by tesselated 

 epithelium, and he even admits that the cells forming this 

 layer may be covered by columnar epithelium ; for inasmuch 

 as the villi project and penetrate into the uterine glands, the 

 columnar epithelium of these glands may still remain adherent 

 to the isolated villi. My own researches upon this point have 

 led to the following results : Some villi really have an invest- 

 ment of columnar cells, but in that case there is no subjacent 

 layer of epithelium. The columnar cells bound the cavity 

 of the villus in which the bloodvessels are contained. The 

 young villi, on the other hand, are invested neither by columnar 

 cells, nor by pavement cells, nor indeed by any well-defined 

 cell bodies. They are rather composed of simple protoplasm, 

 with numerous nuclei imbedded in its substance. The villi, as 

 is well known, shoot out protoplasma-threads or knots from 

 their substance. These elongate, and become thickened, and 

 nuclei accumulate in them ; yet there are not here any well- 

 defined cell groups, but merely a coherent mass of protoplasm. 

 A cavity subsequently forms in the villus, but even then no 

 epithelial cell walls can be brought into view by the silver 

 method of staining. 



The villus however is soon invested by a layer of columnar 

 cells, formed from this mass of protoplasm containing nuclei. 

 At least, it is only in this way that the successive 

 histological stages and phenomena can be explained. In the 



* Zur Histologij der Nachgeburt, (On the histology of the afterbirth,) 

 Hoist's Beitrdge zur Gyndcologie und Geburtskunde, 1867, Heft ii. 

 t R. Wagner, Physiologie, 3rd edition. 



