LARVAL DEVELOPMENT IN THE SEA URCHIN 107 



In the various degrees of differential inhibition described, the 

 progressive obliteration of metabolic gradients, i.e., of axes ap- 

 pears. Bilaterality may be almost completely obliterated, while 

 longitudinal and apico-basal gradients still remain effective (figs. 

 86, 87) ; and in other cases both bilaterality and the longitudinal 

 axis are practically obliterated and the apico-basal axis remains 

 as the chief determining factor in growth and differentiation. 

 This condition is most evident where differential acclimation* or 

 recovery occurs in the apical region, but there is little or no indi- 

 cation of other axes in the larva. Figures 40 to 53 and 57 to 81, 

 show various stages in this axial obliteration. The limit in this 

 direction is the obliteration of all axes, including the apico-basal, 

 axis. This limit is approached or perhaps attained in some 

 cases (figs. 19 to 21). Under these conditions definite progres- 

 sive development and localized differentiation cease, although life 

 may continue until ended by starvation. 



In differential acclimation and differential recovery the changes 

 in metabolic relations between different points of a metabolic 

 gradient are in the opposite direction from those in differential 

 inhibition. Since, with the method employed, differential accli- 

 mation and recovery are possible only after differential inhibi- 

 tion, it may happen that metabolic gradients are so far oblit- 

 erated by differential inhibition that differential acclimation or 

 recovery is impossible, but where the gradient is not obliterated 

 to this extent, differential acclimation and recovery consist in 

 an intensification, a steepening of its slope, beginning at the 

 high end. The metabolic differences between different points of 

 the gradients are increased, and the form and proportions of 

 the larva and the positions of localized differentiations show 

 changes in the opposite direction from those characteristic of 

 differential inhibition. The apical region develops at the ex- 

 pense of the basal, the anterior at the expense of the posterior 

 and the median at the expense of the lateral. 



The differences between the forms resulting from differential 

 inhibition and differential acclimation show to what extent the 

 general form, proportions and localization of parts can be altered 

 and controlled in this way. It is possible to transform the 



