586 J. T. PATTERSON 
uterus and the loosening of the vesicle. However, these stages 
were studied under the high and low powers of the binocular mi- 
scroscope before the uterus had been placed in the fixing fluid, and 
it was therefore possible to make out most of the details of 
structure. 
In general appearance these vesicles are not unlike the largest 
of those which were found lying free within the uterine cavity. 
The large polygonal trophoblastic cells are clearly discernible, 
and the embryonic area appears as a whitish spot lying directly in 
contact with the mucosa. In size, too, they are not larger than 
the more advanced free-vesicles. 
These four vesicles were sectioned for microscopic examination, 
and in three of them a detailed study reveals nothing different 
from what we have already seen in the free stages; but in the fourth, 
which was one of the two that remained attached for the longest 
period, an important difference was observed. The change to 
which I refer involves the most essential part of the vesicle, the 
embryonic ectoderm, and consists of a thickening of that structure 
(fig. 48). Itis certain that this increase is not due alone to a mul- 
tiplication of cells, since mitotic figures are rarely found, but to a 
distinct rounding up of the entire embryonic ectoderm. In fact 
this change is the beginning of a process that will eventually trans- 
form the lens-shaped ectodermal mass into a ball-like structure. 
In the study of these four vesicles particular attention has been 
paid to the area of trophoblast (Rauber’s layer) which directly 
overlies the embryonic ectoderm, and which forms the seat of at- 
tachment. The trophoblastic cells of this area do not as yet be- 
tray any evident changes looking to the formation of the ‘Trager.’ 
We must conclude therefore that for a short time at least the vesi- 
cle is held to the mucosa by adhesion. It would seem that it sim- 
ply ‘sticks’ to the uterine lining. Nor is there any evidence 
to show that there is a localized area within the attachment zone 
of the fundus to which the vesicle migrates before becoming at- 
tached. Any spot on the entire attachment zone may furnish a 
foothold for the vesicle, if one may judge from the collected date 
on the distribution of about twenty young attached stages. 
Apparently the vesicle adheres to the mucosa very soon after it 
