LARVAL DEVELOPMENT IN THE SEA URCHIN 111 



reason to believe that certain of the minor metabolic gradients 

 in the pluteus are not present in the crinoid larva; certainly they 

 are not effective factors in larval development. 



The modifications of form produced by differential inhibition 

 and differential acclimation and recovery, demonstrate the real- 

 ity of the metabolic gradients as effective dynamic factors in the 

 development of Arbacia. The analysis of these modifications 

 indicates that the apical region is the region of highest rate of 

 reaction in the organism; or in other words, that the region of 

 highest rate of reaction becomes the apical region. From the 

 apical region the rate of reaction decreases in all directions, but 

 less rapidly on that side which becomes anterior than on that 

 which becomes posterior, and apparently less rapidly along the 

 median region than laterally. The development of the skele- 

 ton of course modifies these simple relations and determines new 

 localized regions of growth and so new local gradients. 



The method of demonstrating the axial metabolic gradients 

 by the death gradients in lethal concentrations of inhibiting 

 agents (Child, '13 b, '14 a, '15 a, '15 c, Chap. Ill, '16 a) is much 

 less delicate than the method of modifying development by dif- 

 ferential inhibition and differential acclimation or recovery, but 

 the conclusions reached by means of the cruder method, so far 

 as they go, are confirmed, and further conclusions made possible 

 by the more delicate method. 



A few data not yet published on differential inhibition in the 

 larvae of the starfish and of certain polychete annelids, indicate 

 that the metabolic relations can be altered along at least the 

 apico-basal gradient in the same way as in Arbacia. Moreover, 

 in the reconstitution of isolated pieces of Planaria, differential 

 inhibition and differential acclimation and recovery in the longi- 

 tudinal and transverse axial gradients can be brought about 

 in the same way as in the sea urchin larva, and the resulting 

 modifications of form are of the same character. In differen- 

 tial inhibition anterior (apical) regions are most inhibited, pos- 

 terior least, and organs normally bilateral, such as the eyes and 

 the cephalic lobes, approach the median line and, in the more 

 extreme degrees of differential inhibition, become median (Child, 



