CELL-LINEAGE. 



1 -7 



0,3 



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 (23) 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 (4<^^ 4'^^) 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 (i,d^ 

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

 i,d-, as in Discocaelis. 



