Fig. 6, Leptoplana. (Camera drawings from the transparent living embr}-os. ) 

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

 second division of 2 ; O, side view, approximately 60 cells, showing the third 

 ectoblast cell (2^) derived from 2, the fourth quartet (4) and the basal entoblasts 

 (D, C). D, delamination of mesoblast in the fourth division of 2 Tshaded), 

 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 poster- 

 ior cell [of the fourth quartet. J^, posterior view of ensuing stage, showing the 

 two posterior mesoblast cells ( shaded) lying in the interior, and a marked in- 

 equality between (4d!'iand 4^^). F^, later stage; multiplication of the meso- 

 blast-cells (shaded) equality of 4^/^ and ^d^, as in Discocaiis, 



