DEVELOPMENT OF THE ALBINO RAT 303 
forming the roof of the vesicle, not shown in the figure, and 
known as the parietal or transitory ectoderm. In the floor or 
germ disc, there is evident a single layer of cells bordering the 
segmentation cavity or blastocele and possessing a more gran- 
ular protoplasm, which stains a little more intensely in Congo 
red. Their differentiation and characteristic reaction to stain- 
ing agents is at this stage of development not quite so distinct 
as in slightly older stages. This layer of cells, similar to that 
described by Sobotta for the blastodermic vesicle of the mouse 
in essentially the same stage of development, he has termed the 
yolk entoderm, ‘Dotter entoderm,’ a designation which is here 
followed. In the more superficial layer or layers of cells no 
characteristic differentiation is observed. In no portion of the 
floor of this vesicle was a distinct covering or trophoblast layer 
recognized. 
In the vesicle, a section of which is reproduced in B of this 
figure (rat No. 100, 6 days), the floor or germ disc presents 
essentially the same structure as that shown in A. The vesicle 
shown under B, was also folded, especially its roof, which was 
drawn to one side and was thus not cut through its entire length 
in the section figured. Furthermore, the section chosen for 
drawing does not pass quite through the center of the germ disc, 
but a little nearer to one of its edges, which probably accounts 
for the fact that there is recognized for the greater part only a 
single layer of cells, superimposed over the yolk endoderm, 
which layer is continuous with the parietal or transitory ecto- 
derm forming the roof of the vesicle. The cells forming the 
yolk entoderm constitute a single layer and are quite distinctly 
differentiated; one of the cells shows a mitotic phase. The roof 
of the vesicle formed by the parietal or transitory ectoderm, is 
composed of a single layer of flattened cells with flattened nuclei, 
the form and structure of which is more correctly shown in the 
right half of the roof wall, which in the section is cut transversely, 
while the left half, owing to the folding, is shown as cut obliquely. 
In C of figure 23 (rat No. 99, 6 days), there is shown a greatly 
compressed blastodermic vesicle, taken from a series of cross 
sections of the uterine horn. In this figure there is reproduced 
the fifth of a series of 10 sections of 10 » thickness; therefore, 
