RELATION OF VESICULAR MOLE TO CHORION CARCINOMA 141 
'zellschicht' of Langhans. The trophoblast in its primitive form can only be seen 
in the very early ova of the first weeks of pregnancy. I have never been able to 
find any in ova of more than a month old. In one specimen I have examined, of 
about three weeks old, the transition from trophoblast cells to syncytium is well seen. 
Three layers of cells cover in the central core of the villus, the outer layer is com- 
pletely flattened, the nuclei spindle shaped and crescentic. The protoplasm, in which 
these nuclei lie, stains well with eosin and forms a continuous ground work for the 
second layer of nuclei, which are larger and more oval in shape, possessing numerous 
granules, but staining less deeply than the outer layer. The third or innermost layer 
has the same continuous protoplasmic basis, and, lying in it, are large round nuclei, 
having a somewhat transparent appearance, possessing fewer granules and arranged in 
a regular manner round the stroma of the villus. 
Writing on this subject, Peters says : ' I have always found, in all my 
preparations, an intimate and genetic connexion between the ' zellschicht ' and 
syncytium, and I was never able to demonstrate a limiting membrane existing 
between the two layers.' 
I, therefore, regard the whole epithelial covering of the ovum, including that 
of the villi, as foetal and ectodermal, and consider those theories which infer a 
maternal origin for the syncytium and a mesodermal origin for the 'zellschicht' as 
fallacious, and for the following reasons : — 
1. The syncytium cannot be formed from the epithelium lining the glands of 
the uterine mucosa, since these have been seen to possess no connexion with the 
ovum in the earliest stages, their epithelium being quite intact, and at no point 
invading or being invaded by the foetal trophoblast. 
1. The syncytium cannot be derived from the endothelium of the maternal 
blood vessels, as it has been proved by many observers that this shows no sign of 
activity during the early weeks of pregnancy. 
3. The syncytium is not derived from decidual cells, as in Peters' ovum the 
connective tissue of the stroma were only beginning to take on a distinct change at 
some little distance from the ovum. 
4. The ' zellschicht ' of Langhans is not a specialized stratum of the con- 
nective tissue cells of the stroma, since it has been proved by Minot, Peters, and others 
that this layer is already well seen before the mesoblastic core of the villus has 
penetrated completely into it. 
As will be seen later, these layers of chorionic epithelium take an active part 
in the formation of vesicular mole, and a still more active and important part in the 
production of chorion carcinoma, so that in discussing the pathology of this latter 
condition, it is of the utmost importance that the foetal and ectodermal origin of 
these cells be kept in mind. 
