72 G. CARL HUBER 



bryonanlage.' The same may perhaps be observed in figure 

 26, plate 14, of Selenka's account. In figures 26, 28, 31, and 33 

 of Christiani's contribution this may be postulated, though his 

 figures are useless for a close comparison. Duval does not figure 

 this stage. Sobotta's ('03) figure 7, and figure 33 of the con- 

 tribution of Melissinos, appear to give a corresponding stage for 

 the mouse, but in neither of these figures is the 'ectodermal 

 node' so clearly depicted as in Grosser's and my own figures, 

 at least not until a somewhat older stage. Figure 6 of Sobotta 

 ('03) may very probably be regarded as representing an inter- 

 mediate stage between that shown in E of figure 23 and in A 

 of figure 24. By a proliferation of the cells of the germinal area 

 as shown in the former figure a stage resembling that shown in 

 Sobotta's figure 6, is readily postulated. That the formation 

 of the ectodermal cells is in part due to rearrangement of the 

 cells of the germinal area I believe to be the case, since cell pro- 

 liferation is not marked in this stage. The enlargement of the 

 more peripheral cells of the germinal area, leading to the anlage 

 of the ectoplacental cone, would of necessity cause the forming 

 ectodermal node to force the yolk entoderm into the cavity of 

 the vesicle, and thus form the anlage of the egg-plug and initiate 

 the phenomenon of entypy of the germ layers. 0. Hertwig, in 

 describing the inversion as observed in the mouse and rat, after 

 considering the formation of the Trager through proliferation of 

 the cells of the Deckschicht, following here Selenka's account, 

 states, referring to the Trager, "Durch ihn wird der formative 

 Teil des Ektoblasts nach dem Centrum der Blase vorgetrieben, 

 wobei er sich in eine allseits abgegrenzte Epithelkugel umwan- 

 delt." And again, in referring to the development of the guinea- 

 pig, he states: "Wie bei Maus und Ratte zieht sich das forma- 

 tive Ektoderm zu einer Epithelkugel zusammen." Hertwig 

 thus appears to regard the formation of the 'Epithelkugel,' 

 the ectodermal node, as in part at least developed owing to a 

 rearrangement of the cells of the germinal disc. After the 

 formation of the egg-plug or egg-cylinder that portion of the yolk 

 entoderm which covers it is designated by Sobotta as the visceral 

 layer of the entoderm. The scattered entodermal cells, attached 



