80 CM. CHILD 



tions of differential inhibition and differential acclimation ap- 

 pear. The form shown in figure 38, for example, is, in its basal 

 aspect (A); a characteristic case of differential inhibition, re- 

 sembling figure 12 A, with the inhibition decreasing from the 

 anterior to the posterior end, so that the body is relatively nar- 

 rower and the angle of divergence of the skeletal rods less than 

 in the normal (fig. 3 A). 



The side view of this larva, fig. 38 B, is very, different from 

 that of the differential inhibition, figure 12 B. The oral lobe is 

 large, with a rounded knob on its apical end, and the ciliated 

 band develops only in this apical region. Great elongation in 

 the apico-basal axis has occurred, and the enteron is attached 

 at the mouth region, but not at the anal, and the rectal portion 

 is absent. 



Figure 39 is a side view of a form in which differential acclima- 

 tion follows a somewhat greater degree of inhibition and consists 

 chiefly in the apical outgrowth and the broadening of the anterior 

 end. 



The observation of many similar cases has shown that the 

 course of development in cases of this sort is as follows: during 

 the earlier stages of development the action of the agent is di- 

 rectly inhibitory, with the usual differential effect, but, at about 

 the gastrula stage, differential acclimation begins to be appar- 

 ent as a relative acceleration of development in the apical region, 

 as in figures 25 and 26 B. From this apical region the rela- 

 tive acceleration progresses basally, but is more marked in ante- 

 rior than in posterior, and apparently also more in median than 

 in lateral regions. The basal region undergoes acclimation so 

 much less rapidly and less completely than the apical that it 

 shows no secondary changes before development comes to a 

 standstill in the solution. 



When the apical elongation begins the entoderm remains at- 

 tached to the region where the mouth is to form, and it may 

 break its connection with the blastopore region, either through 

 closure of the blastopore (which often occurs in inhibition) or by 

 actual rupture and so is carried apically and remains closed poste- 

 riorly, and the rectal region is absent. In short these larval 



