DEVELOPMENT OF THE ALBINO RAT 315 
observations on the mouse egg as concerns this point. He has 
critically reviewed Selenka’s and Jenkinson’s contentions as 
to the participation of the covering layer in the formation of 
the Trager or ectoplacental cone, reaching the conclusion that 
there is no evidence in support of this. In accord with Duval— 
and in this I concur—he states: ‘‘Die mesometrale Spitze des 
‘Tragers Selenkas’ ist, wie auch Duval richtig bemerkt, sogar 
ganz auffillig arm an Mitosen.’’ The anlage of the ectoplacental 
cone or Trager, it would appear to me, is primarily the result 
of enlargement of its constituent cells, this enlargement of cells 
involving the more peripherally placed cells of the somewhat 
thickened germinal disc. In none of my preparations showing 
early stages in the formation of this structure are mitotic figures 
evident. Grosser in his figures 67 and 113, shows a germinal 
vesicle of the albino rat of 63 days in its normal position in the 
decidual crypt. The vesicle there figured is about identical in 
time and stage of development to those figured by me in figure 
24. In his figures, the Traiger (7’r.) is represented as consist- 
ing of relatively few cells in which no mitoses are evident. In 
slightly older stages after the means of nutrition of the vesicles 
is improved through ingestion of maternal blood cells (Sobotta) 
mitotic figures may be observed in the ectoplacental cone, as 
shown in C of figure 24. In the rat as in Mus sylvaticus and 
the guinea-pig (Selenka) the ectoplacental cone arises as a 
solid mass of cells; in Arvicola arvalis (Kupffer) it is at first a 
hollow structure and is in part formed by invagination; in the 
white mouse (Sobotta) the form of this cell mass may vary greatly 
and may be solid or penetrated by a mere slit or again by a more 
extensive cavity. 
The earlier stages in the formation of the egg-plug or egg- 
cylinder I have not been able to follow. In the youngest stage 
showing this, at my disposal, A of figure 24, it consists of a cen- 
tral node of compactly grouped cells, of polyhedral form, quite 
definitely demarked from the surrounding cells, and very generally 
of oval form. This mass of cells I have designated the ectoder- 
mal node. In Grosser’s figures (67 and 113, e, Hc) an identical 
structure may be observed, designated as ‘Ectoderm der Em- 
