PLATE 1 



EXPLANATION OF FIGURES 



1 Median section of a blastula of Ophiura brevispina in which the mesen- 

 chyme is differentiating. Fixation in sublimate-acetic . Cell outlines are not 

 evident, and the yolk material has been dissolved and removed during dehydra- 

 tion, but the former positions of the yolk spherules are indicated by minute 

 vacuoles. 



2 Part of a cross section of a blastula, fixed in osmic Miiller solutions, show- 

 ing resting and dividing cells containing yolk spherules. 



3 Part of a tangential section of a blastula, fixed in osmic-Muller solutions, 

 showing resting and dividing cells. 



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