70 G. CARL Hl'BER 



stages of rodents presenting the so-called inversions of the germ 

 layers. O. Hertwig in his chapter "Die Lehre der Keimblatter" 

 gives a brief resume 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. In a 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 



