52 Pub. Puget Sound Biol. Sta. Vol. 2, No. 34. 



which lie between the cells of the second quartet, these latter 8 cells having 

 now come to lie nearly on a level. 



Either just preceding or just subsequent to this division the posterior 

 macromere (D) divides laeotropically giving rise to a cell (^-D) which is 

 apparently homologus to that found in Crepidula (Conklin 1897) and Um- 

 brella (Heymons 1893). A 29-cellcd stage thus occurs, but the writer has 

 not been able thus far to follow accurately the steps in its formation, nor to 

 determine the time relation between the formation of ^D and the second 

 division of the first quartet; hence the cell 4D is not included in Fig 20 

 in which 28 cells appear. 



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