ego MORGAN AND HAZEN. [Vol. XVI. 



zwischen den Entoderm- und Ektodermzellen sichtbar ist. Man 

 konnte freilich den Einwand erheben, dass alle eingestulpten 

 Zellen als Entodermzellen zu bezeichnen sind ; aber dieser 

 Einwand konnte nur auf der vorgefassten Meinung beruhen, 

 dass alles, was nach innen gelangt, als Entoderm zu bezeichnen 

 ist. Ich habe schon oben in Uebereinstimmung mit Hatschek 

 angegeben, dass der Unterschied zwischen den Ektoderm- und 

 Entodermzellen schon im Blastulastadium, also vor der Ein- 

 stiilpung, sich bemerken lasst ; dieser Unterschied kann seine 

 Bedeutung nicht verlieren, wenn es sich ergiebt, dass die Ekto- 

 dermzellen sich an der Einstiilpung auch betheiligen. Die 

 aktive Rolle der Ektodermzellen bei der Einstiilpung und die 

 Betheiligung derselben an der Bildung der dorsalen Wand der 

 Hohle kann auf solche Weise keinem Zweifel unterliegen." ^ 

 In regard to the mechanism of the invagination, Lwoff again 

 dissents from Hatschek's view. He shows that Hatschek's 

 account of the enlarging of the endodermal cells is an insuffi- 

 cient explanation in itself, and, moreover, the inturned cells, 

 instead of enlarging, are reduced in size by cell division. We 

 have found that it is difficult to tell how much the endodermal 

 cells increase in volume after each division. A very slight 

 increase in the size of each cell would be sufficient to greatly 

 increase the area of the cell plate, even if the cells themselves 

 do not after division assume their original size. Lwoff' s state- 

 ment that during gastrulation the endodermal cells almost cease 

 to divide is certainly incorrect. Dividing cells are to be found 

 throughout the entire gastrulation period in the endoderm. 

 Some preparations even show only the endodermal cells divid- 

 ing, and others only the ectodermal. Lwoff has observed the 

 latter only, but an examination of a large number of embryos 

 shows that division takes place in both layers. The number of 

 ectodermal cells is larger than that of the endoderm, so that 

 cell division might be found somewhat more often in the outer 

 layer ; but from this it would not follow that any cell of the 

 ectoderm divided more often than any cell of the endoderm. 

 Further, Lwoff's statement that cell division is more abundant 

 on the dorsal side of the blastopore is certainly incorrect, since 



1 Lwoff, p. 7. 



