306 MINOT. [Vot. II. 
potheses is correct. When we consider the precision and exacti- 
tude of Kastschenko’s observations, which actual specimens 
enable one to verify, there is in my judgment no reason left for 
differing from the conclusion that both layers are parts of the 
foetal ectoderm. | 
Governed by the difficulty of accounting for the presence of 
maternal blood in the intervillous spaces, and therefore appar- 
ently outside the maternal tissues, several investigators have 
been led to seek for at least an endothelium outside the chori- 
onic epithelium. Some authors, as for instance Winkler, have 
asserted the existence of such an endothelium, but after a pro- 
longed and careful search, I fail to find anything of the kind, 
and in this result it seems to me the best observers are agreed. 
The conclusion, I think, may now be safely formulated that 
the chorion is covered externally by the foetal ectoderm, and 
has no other covering in any part except, of course, where the 
chorion lzve rests upon the surface of the decidua, and where 
the tips of the villi touch the serotina; but the morphological 
distinction holds, and the decidua is no more the covering of 
the chorion, than are clothes morphologically the covering of the 
body. I believe further, on grounds stated below, that the con- 
clusion just formulated holds true of the chorion at all periods. 
The further history of the chorionic mesoderm is so fully 
given by Langhans in his invaluable memoir, 110, and Kast- 
schenko, 107, that there is little to be added. In the earliest 
stage I have been able to examine, an ovum of the third week, 
the matrix of the chorionic connective tissue, in a preparation 
stained with cochineal or hematoxyline, and imbedded in paraf- 
fine for cutting, appeared hyaline and glistening, owing to its 
refrangibility (Cut 19); it has lacunze in which the cells lie; the 
cell bodies are either shrunken or colorless, so that lacune, 
except for the staining of their contained nuclei, are clear and 
light. This appearance I find again in specimens a little older. 
The image is entirely distinct from that of the same layer later, 
for then the cells are stained darker than the matrix, which at 
the same time has lost its homogeneous character, and acquired 
a fibrillated look. Very different from my own sections are 
several which I owe to the kindness of Professor Langhans of 
Bern, and which that distinguished investigator informs me are 
from a three-weeks ovum, which had been preserved in osmic 
