CELL-LINEAGE. 



FIG. 4, LEPTOPLANA. (Camera drawings from the transparent living embryos. In these figures 

 the subdivisions of the micromeres are accurately shown.) 



A, 32-cell stage, from the upper pole ; B, 36-cell stage, from the side, showing second division 

 of 2 ; C, side view, approximately 60 cells, showing the third ectoblast cell (2 3 ) derived from 

 2, the fourth quartet (4), and the basal entoblasts (D, C). D, delamination of mesoblast in 

 the fourth division of 2 (shaded), from the lower pole, showing the basal quartet of entomeres 

 (A-D), and the two somewhat unequal cells (^d l , $d z ) formed by the vertical division of the 

 posterior cell of the fourth quartet. , posterior view of ensuing stage, showing the two pos- 

 terior mesoblast cells (shaded) lying in the interior, and a marked inequality between (4^* 

 and 4<tf 2 ). F, later stage ; multiplication of the mesoblast-cells (shaded), equality of ^d l and 

 4</ 2 , as in Discoccelis. 



