DEVELOPMENT OF THE ALBINO RAT 299 
mass into ectodermal and entodermal cells; the outer layer, 
covering layer, Rauber’s cells, or trophoblast, is clearly differ- 
entiated from the inner mass by a distinct space. Duval has 
recognized in early stages of blastodermic vesicle formation of 
the mouse and rat, in the thicker part of the vesicle, entodermal 
and ectodermal cells. The former are of irregular form, pos- 
sess granular protoplasm and are said to possess the property 
of ameboid movement. The remaining cells are recognized 
as ectoderm; a distinct covering layer is not recognized. In 
Christiani’s figures (rat), which are, however, so diagrammatic 
as to be of little value and are evidently drawn from poorly 
fixed material, entodermal cells, ectodermal cells, and covering 
cells may be recognized as per legends. Melissinos (mouse), 
while not describing definitely a peripheral or covering layer, 
states that outer cells of the thicker pole, like the cells enclosing 
the segmentation cavity, stain less deeply than do the more 
centrally placed cells. In earlier stages of vesicle formation, 
neither in figures nor in text as given by this observer, do I find 
reference to a differentiation into ectodermal and entodermal 
cells. The observations of Selenka, Duval, Robinson, Jenkin- 
son, and others, bearing on the structure of the blastodermic 
vesicle of the mouse and the rat in early stages of development 
have been so thoroughly and critically reviewed by Sobotta 
that an extended discussion has here been deemed unnecessary. 
It may here suffice to say that while Sobotta’s observations were 
made and his discussions based on ova obtained from the mouse, 
my own observations made on the albino rat are in agreement 
with his and support the contention that in early stages of blas- 
todermic vesicle formation a differentiation of the thicker part 
or the floor of the vesicle into a covering, Rauber’s cell, or tropho- 
blast layer, and a further differentiation into ectodermal and 
entodermal cells, is not to be made: we differ in our accounts of 
the beginning of the formation of the segmentation cavity. 
An outer or covering layer is suggested in certain of my own prep- 
arations, most clearly in that sketched in E of figure 20. How- 
ever, a uniform difference in structure and reaction to staining 
reagents of the outer layer of cells is not present in my own 
preparations. None of my own preparations gives evidence of 
