No. 2.] THE CELL LINEAGE OF PLANORBIS. 99 



quadrants b and c, three pairs of cells of the third quartette 

 placed the one above the other, while in the posterior quad- 

 rants a and d, there are three pairs of cells similarly arranged 

 situated above the large cells $a 2 and $d z . Before the fore- 

 going divisions of the third quartette are completed the cells 

 2a*' 1 ' 1 , 2& 2 ' 1 ' 1 etc., and 2a 2 ' 2 , 2& 2 ' 2 , etc., have begun to divide, and 

 there soon follows a cleavage of the four cells at the apical 

 pole (ia 1 ' 1 , \b l '\ etc.). Meanwhile the fourth quartette has 

 divided again, making sixteen entomeres in all. With the 



FIG. 3. Gastrula with the blastopore nearly closed ; /', prototroch. 



cleavage of the basal cells of the arms of the cross the num- 

 ber of blastomeres in the egg reaches one hundred and four. 

 The cells of the third quartette continue to divide quite 

 rapidly, while the cleavage of the second quartette takes place 

 more slowly. The cells of the first quartette whose divisions 

 have hitherto been of the spiral type begin, at the next cleav- 

 age, a series of bilateral divisions which result in a longitudinal 

 splitting of the arms of the cross similar to that which was 

 found by Conklin to occur in Crepidula. This splitting first 

 occurs in the anterior arm of the cross ; somewhat later the 

 lateral arms begin to divide ; the posterior arm, as in Crepi- 

 dula, remains undivided. The cross develops much more 

 slowly than in the form studied by Conklin. The details of 



