THE EMBRYONIC RUDIMENT 
25 
it is of very minute size the formation is a closed sac, and not an 
accidental grouping of the mesoblast cells. It is important to note that 
while the larger vesicle is attached definitely only at one point, the 
smaller is closely surrounded by mesoblast strands. These are absent only 
in one section, and on the side looking towards the position in the 
central space which is occupied in other sections by the larger vesicle. 
Figure III. Drawing op a Reconstruction Wax Model of the Blastocyst. 
143 
1 ‘ 
The outer darker layer is the cyto-trophoblast; the inner lighter, the meso¬ 
blast represented as if solid. The irregular cavity is the retraction cavity in 
the mesoblast. It contains two vesicles : the larger is the torn and collapsed 
amnio-embryonic vesicle, the smaller is the entodermic vesicle. 
The model was constructed by building up wax plates to represent what was 
really a cast of the cavity of the blastocyst, with a space in its centre containing 
the vesicles. The model was then cut across so as to present to view the portion 
of the retraction cavity containing both vesicles. This portion of the model was 
then cast in plaster of Paris, and an outer case constructed which represents in 
a diagrammatic fashion the cellular layer of the blastocyst wall. 
The model (Figure hi) shows the relative sizes of the two sacs, and 
their position in regard to one another. The space between the two is 
obviously considerable, but as it contained no mesoblast it is probable 
that there has been a certain amount of accidental displacement. The 
appearances strongly suggest that the larger vesicle has suffered most in 
the shrinking of the tissue surrounding it. It seems to have collapsed 
and drawn away from the smaller vesicle. Two forces must have operated 
D 
