No. 2.] THE CELL LINEAGE OF PLAA'ORBIS. 97 



The divisions of the four cells of the third quartette and both 

 tiers of the first quartette quickly follow. The pairs of cells 

 belonging to the third quartette are arranged radially as in 

 Limax (Kofoid), the one cell lying above the other. After 

 these divisions the cross makes its appearance, becoming a 

 very conspicuous feature of the egg. The cells of which it is 

 composed are the eight cells resulting from the laeotropic divi- 

 sion of the apical cells of the first generation of ectomeres 

 and the four upper cells of the second quartette which form 

 the tips of the arms. The angles between the arms of the 



id" 



FIG. 2. Egg of sixty-four cells, seen from above. 



cross are occupied by the four pairs of trochoblasts (itf 2 ' 1 , 

 i a", ib 2 ' 1 , \b 2 ' 2 , etc.). Compared with Crepidula (Conklin) 

 and Umbrella (Heymons), in which the trochoblasts divide 

 only at a very late period of cleavage, the division of these 

 cells occurs remarkably early. In this respect Planorbis agrees 

 much more closely with Limax (Kofoid), in which the corre- 

 sponding cleavage occurs at near the forty-cell stage. Both 

 in Crepidula and in Umbrella the trochoblasts are small, but 

 in Planorbis they are conspicuous from their large size and 

 transparency. 



The next cells to undergo division are the three entomeres 

 A, B, and C, the corresponding division of D having taken 

 place at an earlier period. This cleavage is dexiotropic, the 

 cells at the vegetative pole being much smaller than those of 



