DEVELOPMENT OF SPELERPES BILINEATUS 



227 



ences in judgment or of observation as to whether a given mark should 

 be recorded or not. If a mark was faint it might be recorded one day 

 and the next considered too faint to record. 



Figs. 56^3 represent the changes observed in an egg, whose 

 lower hemisphere was studied in the way just described. The 

 upper hemisphere is shown in the upper row, the lower in the lower 

 row. The movements of the marks are so clearly shown that a 

 detailed description is not necessary. It should, however, be 



Figs. 56-63 



noted that in the region dorsal to the blastopore, the marks are 

 too far above it to be drawn into it. In figs. 62-63 the stain 

 has faded so badly that it is no longer of much value. 



The interpretation of the movements described above is not 

 difficult. As already stated, Nile blue sulphate stains the yolk 

 granules. Hence the extension of the marks denotes cell division 

 followed by an active migration of some of the parts towards the 

 blastopore. The region of this cellular activity lies in a band 

 about the equator. The cell activity may begin above the dor- 

 sal lip slightly in advance of that on the opposite side of the egg, 

 since it has been noted sometimes, that marks lying above the 

 dorsal lip begin to extend somewhat before those of the opposite 

 side. Aided by this start, and the nearness of the blastopore, 



