298 G. CARL HUBER 
figure 20 and A of figure 22. According to this observer, the 
segmentation cavity arises as a single space, due to an accumula- 
tion of fluid secreted by the cells of the morula. This secretion 
is evidenced by globule-like droplets which are shown in his 
drawings as adhering to certain of the cells bounding the seg- 
mentation cavity. In my own preparations of the albino rat, 
I find no evidence which would lead to the supposition that the 
segmentation cavity does not begin as a single space nor do I 
find any evidence of secretory globules as described by Melis- 
sinos. Selenka has described quite fully two blastodermic 
vesicles of the mouse, lying free in the uterine lumen. His 
account of their structure, supported by two somewhat dia- 
gramimatic figures, is as follows: The wall of the vesicle is formed 
by a layer of covering cells—‘ Deckzellen’—constituting a cover- 
ing layer—‘‘Deckschicht or Rauber’s layer.’’ The space en- 
closed by this layer, for about one-third to one-half of its extent, 
contains the ‘formative cells,’ for the remainder it contains 
fluid. The covering cells and formative cells are said to be sep- 
arated by a sharp line. The formative cells are in all parts 
separable into two fundamental germ layers. An inner layer, 
bordering the cavity and constituting the entoderm, is said to 
be composed of cells possessing more deeply staining nuclei, 
irregular outline, with tongue-like processes which extend into 
the cavity, and a granular protoplasm; further, of cells which 
are more clear, more peripherally placed, and which constitute 
the ectoderm. Each of these fundamental germ layers con- 
sists of a single layer of irregularly formed cells. According to 
the observations of Selenka, therefore, the floor of the vesicle 
consists of three layers of cells; an outer covering layer—‘Deck- 
schicht’ or ‘Rauber’s layer’—an inner layer of entodermal cells, 
and an intermediate layer of ectodermal cells. Jenkinson’s 
account reads as follows: ‘““There are present (1) an outer layer, 
one cell deep, of trophoblast, which is continuous over (2) an 
inner mass which becomes differentiated into the embryonic 
epiblast and the hypoblast, and which is quite distinct from 
the overlying trophoblast, as my specimens invariably show.” 
In Jenkinson’s figures 1 and 2, giving early stages of the blasto- 
dermic vesicle, there is not shown a differentiation of the inner 
