PATHOLOGIC OVA, ALBINO RAT 125 



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 crypt which 

 communicates as yet freely with the lumen of the uterus. 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 is 

 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 M 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- 



