242 



MEAD. 



[Vol. XIII. 



The cell X^ divides meridionally and bilaterally, and the 

 resulting cells, equal in size, lie one on either side of the 



:Attt'* 



Fig. IV. — Diagram of a group of somatic-plate cells. The perpendicular lines indicate the sagittal 

 plane of the trochophore. Figs, a and b show the origin of the bilateral group of cells in c, 

 where I', 11', IIP on the left correspond in position to I', II', III'' on the right, although the 

 sagittal plane passes through I'. After the subsequent divisions the group is still bilateral, but 

 the descendants of the cells I', II', and III' no longer correspond in position to those of I', II', 

 and III'. The four cells in e, whose nuclei are indicated, constitute the paratroch. 



median line of the embryo. The later divisions of these cells 

 are strictly bilateral as far as I have followed them (Figs. 39, 

 40-52, cf. p. 245). 



