DEVELOPMENT OF THE ALBINO HAT <>'.> 



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

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

 ers or en typy of the germ layers. "Inversion of the germ layers 

 Blatterumkehrung" in the ova of rodent- was probably firsl 

 recognized by Reicheri 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 ilie 

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

 white rat and the mouse. Selenka gave thi< 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 ra1 . 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 follow-: 



Wahrend beidem Kaninchenei, nach erfolgter Sonderung der forma- 

 tivcn Furchungszellen in ausseres Ektoderm mid inneres Entoderm, 

 die gesammste Lage der ausseren DeckzelleB zu einer diinnen resisten- 

 ten Membran zusammenschrumpft, verdickl sich bei den Nagern 

 mit invertirten Keimblattern der mil den formativen Zellen in Con- 

 t;ici befindliche Abschnitl der Deckschichl unter lebhafter Zellver- 

 mehrung zu einem spharischen oder konischen Gebilde, welches ich als 

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



ins Innere der Keimblase ha1 zur Folge, dass die scheibenformigen 

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

 chen zu zwei concentrischer HohJkugeln erweitern, sondern, ehe sic 

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

 geschofen, vorgestulpl und damit invertirt werden. 



In a later publication, this observer also suggested the name 

 'Entypie do^ Keimfeldes' as a more comprehensive term than 

 'Blatterumkehrung' under which may he included type- with 

 inversion of the germ field without actual inversion of the germ 

 layer-. In later years Duval, Christ iani. Robinson, .Jenkinson, 

 Sobotta, Kolster, D'Erchia, Spee, Burckhard, Melissinos, A\'ida- 

 kowich, Lee and others have studied the earlier developmental 



