No. 3.] UTERUS AND EMBRYO. 401 
dency to fibrillar differentiation in quite a wide zone around the 
blood-vessels; in this zone the cells become elongated and 
irregularly fusiform; around the larger vessels the cells are 
grouped in lamina, making the structure similar to that already 
described in the walls of the vessels of the umbilical cord; after 
the perivascular coats acquire a certain thickness, the cells of 
the inner layers are more elongated, more regularly fusiform, 
and more closely packed than those of the outer layer; the 
transition from the denser to the looser tissue is gradual. We 
are perhaps entitled to recognize in the denser inner layer the 
media, in the outer looser layer the adventitia, although neither 
of the layers has by any means the full histological differentia- 
tion characteristic of the like-named layers of the blood-vessels 
of the adult. 
The epithelium of the chorion becomes differentiated in three 
different ways: 1°, upon the chorion frondosum; 2°, upon the 
chorion lave; 3°, upon the villi. For a correct knowledge of 
the remarkable changes which the epithelium undergoes, partic- 
ularly in the placenta, we are indebted to the remarkably exact 
investigations of Langhans, 110 and 111. This author left two 
points of importance unsettled; namely, the origin of his 
“ Zellschicht,’ and of the “canalisirtes Fibrin.’ Kastschenko 
has traced the cellular layer (Zel/schicht) to the epithelium, as 
already stated: compare pp. 463-469 of his memoir, 107. My 
own observations show, I think conclusively, that the canalized 
fibrine arises through a degenerative metamorphosis of the epi- 
thelium, which begins in the outer layer and may invade the 
inner layer (Langhans’ Ze//schicht). Let us consider separately 
the three series of modifications of the chorionic. ectoderm. 
In the region of the chorion frondosum the inner layer of the 
ectoderm (the cellular layer of Langhans) becomes irregularly 
thickened in patches, which present every possible degree of 
variation as to number and as to their breadth and thickness. 
Although at first the cellular layer is more or less continuous 
and composed of uniform cells, this is not the case in later 
stages. We must assume that with the growth of the mem- 
brane the epithelium increases in area, but remains in many 
places single layered, developing no “ Zellschicht.” The patches 
of cells have been well described by Langhans, 110, and Kast- 
schenko, 107, 466, and are represented with lower power in 
