PATHOLOGIC OVA, ALBINO RAT 369 
24, Part I.» Reference to this figure may serve to show that dur- 
ing the early hours of the seventh day after insemination, the 
phenomenon of inversion or entypy of the germ layers is initiated 
in the albino rat. The ova are, on reaching this stage of develop- 
ment, enclosed within a well differentiated decidual erypt which 
communicates as yet freely with the lumen of theuterus. These 
crypts present a continuous lining of uterine epithelium; the con- 
tained ova are thus not as yet in direct relation with the ma- 
ternal decidua. In the normal blastodermic vesicle of this stage, 
the ectoplacental cone is in anlage, and in the cell mass which 
extends into the cavity of the vesicle—the egg-plug or egg-cylin- 
der—there is evident a clearly circumscribed nodule of cells, 
which has been designated the ectodermal node and recognized 
as the anlage of the primary embryonic ectoderm; this node is in 
part surrounded by the yolk entoderm. In the uterus of rat No. 
54, there are contained nine blastodermic vesicles, one of which 1s 
sketched in C of figure 24, Part I. Not nearly all of these ves- 
icles are so favorably cut as that shown in this figure, the ma- 
jority being cut in a plane which is oblique to the long axis of 
the vesicle. However, in all of them the ectoplacental cone and 
the ectodermal node may be determined except in the one shown 
in C of figure 4. This vesicle was obtained from a series of sec- 
tions passing at right angles to the plane of the mesometrium. 
It lies free in a deep decidual crypt and passes through six sec- 
tions of 10 » thickness; thus is compressed from side to side. 
This vesicle is distinctly smaller than the normal ones taken 
from this series, especially so as concerns its cavity. An ecto- 
placental cone is not clearly differentiated, and it is not possible 
to determine an ectodermal node, nor is it clear that the yolk 
entoderm has differentiated. In the cell mass from which ecto- 
placental cone and ectodermal node should have developed, the 
upper portion of this figure, there are evident, in the sections 
figured, four relatively large cells with relatively large nuclei, 
cells which have been interpreted as evidencing retarded seg- 
mentation with consequent retardation in the normal differen- 
tiation of the vesicle. On tracing this vesicle through the series 
of six sections it would seem that the direction of section is favor- 
