46 ART. 3. — S. IKEDA : CONTRIBUTIONS 



and the anti-blastoporic regions of an egg in the stage of Fig. 

 52. In each of these figures, although the lower part of the 

 zonal mass passes over gradually to the yolk mass, it is in its 

 upper part sharply marked off by a line from the yolk cells, as 

 if the cells had moved down from the roof of the segmentation 

 cavity with which they form one continuous sheet, and accum- 

 mulated in a mass here. 



In Fig. 63, the first trace of the blastoporic slit interrupts 

 the region of transition from the equatorial zone to the yolk-mass. 

 In other words the cells directly above the first blastoporic slit 

 are not the true zonal cells, but forms transitional between them 

 and the yolk cells. Such a state of things leads me to accept 

 the statement of Morgan and Tsuda that " the blastopore makes 

 its first appearance on the less pigmented and further developed 

 side of the egg, and, moreover, at a short distance only from the 

 group of large cells around the lower pole," (p. 381), although 

 there is no pigment in my materials. These transitional cells are 

 however doubtless soon transformed into the zonal cells which 

 are in their turn differentiated into the permanent tissue cells 

 roofing over the Archenteron. This process of transformation no 

 doubt goes on all around the lower edge of the equatorial zone, 

 while the cells themselves multiply also by division in situ. As 

 the result of these formative changes, the equatorial zone spreads 

 downward toward the yolk pole. I am therefore able to agree 

 with Assheton ('99), when he says : " In this way there is a 

 gradual apparent creeping of small (black) cells over the surface 

 of the egg — though in reality it is conversion of large cells into 

 smaller in situ as, I believe, is now generally accepted," (p. 227). 



3). The equatorial zone is, however, only a temporary struct- 

 ure. The blastopore lip extends gradually on each side from the 



