272 EARLY ECHINODERM DEVELOPMENT 



ing to Gustafson and Lenicque (1952), that these particles are 

 arranged in a gradient, decreasing toward the vegetal pole. 



4. One of the features of the original double gradient concept 

 of differentiation in the sea urchin is the postulation of animal 

 and vegetal substances, the concentrations of which are highest 

 at the respective poles, decreasing toward the opposite pole. Ac- 

 cording to Gustafson (1954), agents producing animalization of 

 the embryo act by suppressing a mitochondrial inhibitor in the 

 vegetal region, producing larvae with a uniform distribution of 

 mitochondria. Conversely, vegetalizing agents suppress the ani- 

 mal substance which favors mitochondrial production; hence the 

 number of mitochondria in vegetalized larvae is lower than in 

 normal ones. 



5. The primary mitochondrial gradient in mesenchyme blastu- 

 lae is soon succeeded, however, by a series of localized waves of 

 mitochondrial production in actively differentiating regions, 

 which in turn produce substances inhibitory to mitochondrial 

 production in immediately adjacent regions. When the effect of 

 these inhibitors declines, succeeding waves of mitochondrial pro- 

 duction occur in the next regions to differentiate. Gustafson and 

 Lenicque (1952) state: ". . . the localized development of mito- 

 chondrial populations appears to be controlled by inhibitors, pro- 

 duced in regions of high mitochondrial activity." 



It has been noted in the preceding section that the distribution 

 of cytoplasmic inclusions along gradients in unfertilized, or just 

 fertilized, eggs, can have no dii-ect connection with the differen- 

 tiation potentialities of the regions along the presumptive em- 

 bryonic axis, since the disarrangement of particles by centrifuga- 

 tion appears not to affect the subsequent formation of the embryo. 

 But Harvey (1946) reports mitochondria present in the plutei 

 developed from clear egg quarters in Arbacia, although she states 

 that they were not found in day-old blastiilae. In view of the im- 

 portance for primaiy differentiation attril^uted by Gustafson to a 

 gradient pattern of cells in mesenchyme blastulae and young gas- 

 trulae, with reference to their mitochondrial content, a brief con- 

 sideration of the results of other investigators on this subject 

 seems of interest. 



