588 MORGAN AND HAZEN [Vol. XVI. 



large yolk-bearing cells in the endoderm after invagination 

 than in the flattened endodermal plate of the blastula. Lwoff 

 points out the insufficiency of the mechanism proposed by 

 Hatschek and claims that other cells beside the large yolk- 

 bearing ones are also turned in, and in this way he accounts for 

 a sufficient number of cells to fill the archenteric cavity. We 

 have also tried to show that, at the dorsal side, cells poor in 

 yolk are invaginated, although we prefer to speak of these 

 cells as endodermal and not as ectodermal, as Lwoff has 

 done. 



Hatschek's idea of the method of closure of the blastopore 

 is illustrated by our series of text-figures, VI-X. The dorsal 

 lip is supposed to bend around and meet the ventral lip, thus 

 closing the gastrula mouth along the dorsal side of the embryo. 

 Hatschek offers this view, not in a dogmatic spirit, but simply 

 as more in line with his own observations, admitting, however, 

 that Kowalevski's view may be the correct one. Hatschek 

 noticed the early asymmetry of the gastrula, but a comparison 

 of his Fig. 24 with our PL XXXHI, Fig. 2 shows that what he 

 has identified as the dorsal (anterior) lip of the blastopore is, in 

 our estimation, the ventral lip. The older stages, however, are 

 oriented in the same way as are our own. Hatschek noticed 

 that the transition from ectoderm to endoderm is sharpest on 

 the ventral side, and at that point in the later stages he located 

 the two historic pole cells. 



It is not entirely clear to us how Hatschek imagined the 

 backward growth of the dorsal lip of the blastopore to take 

 place. His figures lead us to suppose that the result is, in 

 part at any rate, produced by an increase in the number of 

 cells of this region. The ventral wall remains unchanged, and 

 the ventral lip bends around only so far as it takes part in the 

 reduction of the blastopore. 



Lwoff draws a sharp distinction between ectoderm and endo- 

 derm ; the latter cells being characterized by their size and the 

 amount of yolk contained in them, and he believes that the 

 difference is present even in the blastula stage, so that " ehe 

 die Einstiilpung beginnt, dass also die Sonderung der zwei 

 primaren Keimschichten — des Ektoderms und Entoderms — 



