LARVAL DEVELOPMENT IN THE SEA URCHIN 83 



an apical elongated region (fig. 50); an apical outgrowth may 

 occur and mouth and oesophagus fail to differentiate (fig. 51); 

 or mouth and oesophagus may develop in the absence of an apical 

 outgrowth (figs. 52, 53). Usually in these cases theenteron is not 

 connected with the blastopore region, and the rectal portion does 

 not differentiate (figs. 49 to 51, 53), but occasionally in acids 

 complete differentiation of the entoderm has been observed (fig. 

 52). Except for the case of figure 52, which has been seen only 

 in acclimation to acids, these forms occur in both alcohol and 

 acids. 



In these cases of limited acclimation following a rather high 

 degree of differential inhibition (figs. 40'to 53), the antero-poste- 

 rior axis and bilaterality have been almost or quite obliterated 

 as effective factors in development and, except for the position 

 of the mouth in some cases (figs. 40 and 49), do not appear in 

 differential acclimation. The larvae remain almost completely 

 radially symmetrical and move with the apical end in advance 

 as long as they live. Only the apico-basal axis remains effective, 

 and the metabolic gradient, which constitutes this axis, has been 

 levelled down to such an extent that it is much less effective in 

 determining and localizing differentiation than under the usual 

 conditions. These partial acclimations, as well as the more 

 extreme differential inhibitions, show very clearly that the 

 minor axial gradients can be leveled down and obliterated to 

 such an extent that they do not reappear, while the apico-basal 

 gradient may still remain effective to some extent. This can 

 only mean that these minor metabolic gradients are less per- 

 manently recorded in the protoplasmic substratum (Child, '15 c, 

 pp. 33-35), and this in turn must mean that the high ends of 

 these minor gradients represent lower metabolic levels than the 

 high end of the apico-basal gradient, i.e., the apical region. 

 This conclusion is in agreement with all the facts. If the apical 

 region is the region of highest metabolic rate in the individual, 

 it follows of course that the high ends of gradients in other direc- 

 tions must have a lower rate than the high end of the apico- 

 basal gradient; moreover, the death gradient is much more dis- 

 tinct in the apico-basal axis than in other directions (Child, 



