138 THOMPSON YATES AND JOHNSTON LABORATORIES REPORT 
Having thus briefly described the appearance of the chorionic vesicle in the 
early ovum, the theories concerning the origin of the different layers must be 
considered. This is of particular importance in view of what follows in the discussion 
of chorion carcinoma or deciduoma malignum so called. 
For many years a controversy has been waged by pathologists, Continental, 
American, and British, on this subject, without any consensus of opinion having 
been arrived at. 
On one point, however, all observers are agreed, namely, 011 a mesoblastic 
origin for the central core of the villus, but it is the epithelial or external covering 
to this core which has given rise to so much discussion. 
In a fully-formed villus of the second or third week we find an outer 
plasmodial layer, composed of highly refracting granular protoplasm, taking on a 
deep rich stain with eosin, having no cell walls and well supplied with vacuoles. 
In it are seen numerous darkly-stained nuclei, of various sizes and shape ; some 
are elongated and flattened as though subjected to pressure from without, and lie 
scattered throughout the protoplasm, others are crescentic or triangular, while 
others again are oval or round. These may He massed together in great profusion, 
or they may, in places, be very sparsely distributed ; this layer is the syncytium. In 
direct contact with the foregoing is another layer, composed of more transparent 
cubical cells, having a definite cell outline, containing a single nucleus and separated 
by a slight interval from the mesoblast forming the central core. To this layer 
little attention was paid by early observers, and it was not until 1882 that 
Langhans published his classic description of the Zellschicht. 
Speaking of the distinctive points between this layer and what he called the 
epithelial (syncytial) layer, he says : 'The "zellschicht" consists for the most part of 
polyhedral sharply defined cells, whose lines of division can in most places be 
distinguished, except where the layer is thin, containing clear, almost transparent 
protoplasm, poor in granules, with large, spherical, seldom flattened nuclei and 
usually a nucleolus.' Langhans further emphasizes the variety of forms these cells 
may assume, from tall cylindrical rods to small spherical or flattened cells, according 
to the thickness of the layer and the pressure to which it is subjected. Of the 
syncytium he writes, that it lies closely over the 'zellschicht,' filling up all depres- 
sions in its upper surface, and sending down processes in between the cells, giving 
one the impression that there is here a double layer of epithelium rather than 
two layers of really different tissues. 
Langhans thus believes that the epithelial layer and the ' zellschicht ' are derived 
from two distinct sources, since he plainly states that the 'epithelial' layer is 
derived from the foetal ectoderm, while the ' zellschicht ' has its origin in the 
connective tissue cells of the stroma of the villus, and is, therefore, of mesoblastic 
origin. 
