306 G. CARL HUBER 
section figured, onto the layer of parietal ectoderm. Selenka 
and Duval, who regard the cells of the primary entoderm as 
having ameboid properties, are disposed to regard the entodermal 
cells found lining the parietal ectoderm as having wandered 
from their seat of origin to the side wall of the vesicle. Sobotta 
sees no evidence of such wandering of the primary or yolk ento- 
dermal cells, but suggests that they are drawn to their position 
on the wall of the vesicle during its increase in size; their wander- 
ing, therefore, is more relative than absolute. Certain cells 
nearer the edge of the yolk entoderm, having attachment to the 
parietal ectoderm, which attachment they retain as the vesicle 
enlarges, are thought to be drawn from their close relation to 
the yolk entoderm and to appear as scattered cells lining the 
parietal ectoderm. Now and then, such cells may divide, re- 
sulting in further distribution. Sobotta’s suggestion seems to 
me to be more in accord with the observed facts. In vesicle 
D, the roof, consisting of a single layer of flattened, parietal 
ectodermal cells, presents several major folds as well as minor 
folds. The latter particularly account for the variation in 
thickness of the wall of the vesicle as seen in sections. At the 
lower left of the figure is seen a portion of the wall as seen cut 
on the flat, the shape of the two nuclei here shown as seen in 
surface view may be compared with the long ovoid form of 
similar nuclei when seen in cross section. 
Vesicle E of figure 23 (rat No. 100, 6 days) presents a stage 
that is slightly older than the other four vesicles shown in this 
figure. The floor of this vesicle, the germinal disc, as seen in 
cross section, presents the form of a triangle with its base rest- 
ing on the cavity, the blastocele. When compared with the 
slightly younger stages this portion of the vesicle presents an 
increase in the number of constituent cells, arranged in irregular 
layers to the number of five in its thickest portion. The thicken- 
ing is no doubt in part due to the slight lateral compression of the 
vesicle, but this does not wholly account for it. The cells con- 
stituting this thickened germinal disc are for the main of irregular 
polyhedral form with relatively large nuclei rich in chromatin. 
A distinct covering layer is not evident. On its under sur- 
face there is found a single layer of cells of yolk entoderm. The 
