314 G. CARL HUBER 
stages of rodents presenting the so-called inversions of the germ 
layers. O. Hertwig in his chapter ‘‘Die Lehre der Keimblitter”’ 
gives a brief résumé of our knowledge of the inversion of the 
germ layers as observed in certain rodents, noting that three main 
modifications are to be observed. The first and simplest, as found 
in the field mouse; the second or intermediate as found in the 
rat and mouse; the third and most complex as observed in the 
guinea-pig. Hertwig’s account is based largely on the observa- 
tions of Selenka, the accuracy of which is now questioned from 
many sides. 
My own conclusions concerning the early stages of the entypy 
of the germ layers in the albino rat are made on stages which 
do not portray the very beginning of this process. The vesicles 
shown in figure 24, in which this process is well initiated, however, 
present appearances, on the basis of which certain conclusions 
may be drawn. It is the contention of Selenka that the Trager 
or ectoplacental cone is developed as a result of proliferation of 
covering or Rauber’s cells, superimposed on the formative cells 
of the germ disc. He is followed in this view by Jenkinson, 
who states that “‘At a certain stage this proximal trophoblast 
(the so-called Rauber’s cells of the rabbit) certainly becomes very 
thin, but it never wholly disappears, and soon thickens again to 
form the Trager, or, to use a modern expression, trophoblastic 
syncytium, which is destined to play an all-important part in 
the formation of the placenta.”’ The account of Melissinos is 
difficult to follow, owing to his application of the term ‘Rauber- 
sche Schicht.’ The outer layer of the blastocyst in the region 
of the germinal disc is said to have a transitory existence and to 
disappear almost completely in the earlier stages of blastocyst 
formation. Ina later paragraph he states, ‘“‘dass nur die Rauber- 
sche Schicht existiert und sogar in den folgenden Stadien mit 
zahlreicheren Kernteilungsfiguren, und dass sie den Placentar- 
conus liefert.’”? Attention has previously and on a number of 
occasions been called to the fact that in the albino rat I have not 
been able to differentiate a distinct covering layer—Deckschicht 
or Rauber’s Schicht (Selenka); trophoblast layer (Jenkinson)— 
and have expressed myself as wholly in accord with Sobotta’s 
