304 G. CARL HUBER 
the third dimension of the vesicle is approximately 100 u. It 
is evident that had this vesicle been cut in a favorable plane at 
right angles to the present series, or parallel to the mesometrial 
plane, its form would have approached that of a circle. IJ have in 
my possession one vesicle of this stage of devlopment, similarly 
compressed, cut parallel to the plane of compression, in which 
almost the entire roof falls within a single section of 10 » thick- 
ness. The structure of the vesicle shown in C is very similar 
to that shown in A and B of this figure. The normal form of this 
vesicle is quite readily reconstructed from a study of the series 
of sections into which it has been cut. The cells of the yolk 
entoderm are evident. The parietal or transitory ectoderm 
constituting the roof consists of a single layer of much flattened 
cells, with relatively few nuclei, having, as seen in cross section, 
a long ovoid form, which, when seen in surface view present a 
regular, nearly circular outline (see lowermost nucleus in the 
figure). In similarly compressed vesicles cut parallel to the 
plane of compression, the germ disc may appear as consisting 
of three to four layers of cells. In an imaginary section passing 
in a plane at right angles to that figured in C, and having perhaps 
a slight obliquity, the germ disc would appear as if much thicker 
than that shown in A and B of this figure. Such sections may 
readily lead to false conclusions. 
It seems evident from a study of the material at my disposal 
that during the sixth day after the beginning of insemination 
in the albino rat, the blastodermic vesicle or blastocyst, which 
has its anlage in the latter part of the fifth day, enlarges relatively 
rapidly; this largely owing to a distension of the segmentation 
cavity or blastocele. This enlargement is accompanied by a 
flattening and extension of the enclosing roof cells and by a re- 
arrangement of the cells of the floor, which is reduced in thick- 
ness to a discoidal area, the germinal dise or germ area, forming 
about one-fifth to one-sixth of the wall of the vesicle and con- 
sisting of two or three layers of cells. During the rearrangement 
of the cells which constitute the floor of the vesicle, those adjacent 
to the segmentation cavity or blastocele differentiate to form the 
anlage of the yolk entoderm. The remaining cells of the ger- 
