DEVELOPMENT OF THE ALBINO RAT 12 
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 aryalis, 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 ‘“Triager’ 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 dusseres Ektoderm und inneres Entoderm, 
die gesammste Lage der fusseren Deckzellen zu einer diinnen resisten- 
ten Membran zusammenschrumpft, verdickt sich bei den Nagern 
mit invertirten Keimblaittern der mit den formativen Zellen in Con- 
tact befindliche Abschnitt der Deckschicht unter lebhafter Zellver- 
mehrung zu einem sphirischen oder konischen Gebilde, welches ich als 
‘Trager’ bezeichne; * * * * Die Einwucherung dieses Tragers 
ins Innere der Keimblase hat zur Folge, dass die scheibenférmigen 
Grundbliatter (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 
‘Blitterumkehrung’ 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 
