348 G. CARL HUBER 
consideration of older stages, and the making of a number of 
reconstructions, which it is not contemplated to consider now. 
It must suffice to state at this time that in the albino rat, as 
shown by Widakowich and here shown by me, it is possible to 
delineate clearly the primary embryonic ectoderm and to show 
that the first evidence of the mesoderm is found antimesometrial 
to the future amniotic fold and in the region of the future primi- 
tive streak; therefore is mesoderm which I would regard as 
peristomal mesoderm in the sense of C. Rabl, reference to which 
is made by Sobotta in his discussion of this question. It may 
be that the rat offers more suitable material for the elucidation 
of this question than is to be found in the mouse. In the albino 
rat, the anlage of the mescderm is from the sagittal portion of 
the caudal region of the primary embryonic ectederm, the caudal 
part of the future primitive streak and antimesometrial to the 
amniotic fold. Sobotta gives very favorable consideration to 
the observations of Widakowich, touching this question, which 
he regards as “Bei weitem die beste Darstellung des Gegenstan- 
des.’”’ My own observations fully confirm those of Widakowich. 
These questions will receive fuller consideration in a later pub- 
lication dealing with the embryology of the albino rat, carrying 
the development from the time of the anlage of the amniotic fold 
to the stage of embryo form, the material fcr which is at hand. 
In figure 32 are shown cross-sections of the antimesometrial 
portion of three egg-cylinders in the region of the developing 
mesoderm. Sections drawn in A and B, were taken respectively 
from egg-cylinders obtained from the same uterus as was the 
one shown in sagittal section in figure 31, rat No. 34, 8 days, 17 
hours, after insemination; C, from rat No. 41, 8 days, 16 hours, 
after insemination. It is very probable that the series from 
which A of this figure was drawn, is not cut in exactly the cross 
plane. <A study of the series shows, however, that the deviation 
from this plane is not marked. The sections from which this 
figure was drawn pass a little below (antimesometrial) to the 
region of junction of the primary embryonic and extraembry- 
onic ectoderm. ‘To one side, the lower in the figure, the primary 
embryonic ectoderm shows a slight thickening and evidence of 
