DEV-ELOPMENT OF THE ALBINO RAT 69 



rat, as also in a number of other rodents, in that they show the 

 anlage of the phenomenon known as the inversion of the germ lay- 

 ers or entypy of the germ layers. "Inversion of the germ layers 

 Blatterumkehrung" in the ova of rodents was probably first 

 recognized by Reichert in the guinea-pig, mouse, and rat, though 

 it was much more fully and correctly described by Bischoff 

 as observed in the guinea-pig and a little later by Hensen, also 

 in the guinea-pig. Further observations on this phenomenon 

 were recorded by Kupffer in his study of the development of the 

 field mouse, Arvicola arvalis, and by Fraser on the gray and 

 white rat and the mouse. Selenka gave this question special 

 study, and in a number of monographic communications deals 

 with the phenomenon of Blatterumkehrung as observed in three 

 varieties of the mouse, the white rat, and the guinea-pig. Selen- 

 ka's observations have formed the basis for future work on this 

 problem. They have been widely accepted and extensively 

 quoted. It was he who introduced the term 'Trager' to denote 

 the cell mass which results from proliferation of the covering 

 cells. His own words concerning this point read as follows: 



Wahrend bei dem Kaninchenei, nach erfolgter Sonderung der forma- 

 tiven Furchungszellen in ausseres Ektoderm und inneres Entoderm, 

 die gesammste Lage der ausseren Deckzellen zu einer diinnen resisten- 

 ten Membran zusammenschrumpft, verdickt sich bei den Nagern 

 mit invertirten Keimblattern der mit den formativen Zellen in Con- 

 tact befindliche Abschnitt der Deckschicht unter lebhafter Zellver- 

 mehrung zu einem spharischen oder konischen Gebilde, welches ich als 

 'Trager' bezeichne; * * * * Die Einwucherung dieses Tragers 

 ins Innere der Keimblase hat zur Folge, dass die scheibenformigen 

 Grundblatter (Ektoderm und Entoderm) sich nicht wie beim Kanin- 

 chen zu zwei concentrischer Hohlkugeln erweitern, sondern, ehe sie 

 noch zu dieser Gestalt gelangten, ins Centrum der Keimblase vor- 

 geschofen, vorgestiilpt und damit invertirt werden. 



In a later publication, this observer also suggested the name 

 'Entypie des Keimfeldes' as a more comprehensive term than 

 'Blatterumkehrung' under which may be included types with 

 inversion of the germ field without actual inversion of the germ 

 layers. In later years Duval, Christiani, Robinson, Jenkinson, 

 Sobotta, Kolster, D'Erchia, Spee, Burckhard, Melissinos, Wida- 

 kowich, Lee and others have studied the earlier developmental 



