DEVELOPMENT OF THE ALBINO RAT 71 



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 auffallig 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 6^ 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 Trager (TV.) 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, EC) an identical 

 structure may be observed, designated as 'Ectoderm der Em- 



