B 



Fig. 46, ^-£.— Differential dye reduction in late echinoid blastulae. A, B, Strongylo- 

 centrotus franciscanus, with numerals, 1-4, indicating time-order of reduction of different re- 

 gions in B; C-E, S. purpuratus, preceding and during immigration of mesenchyme. Arrows 

 indicate direction of progress of reduction, and an attempt is made to indicate by their lengths 

 the relative rate of spread of reduction from the center of earUest appearance (from Child, 

 1936a). 



