DYNAMICS — MORPHOGENESIS AND INHERITANCE 421 



stages are not wholly identical with those of the fully developed head 

 with its specialized sense organs. The facts indicate that the 

 ganglionic region is primarily the region of highest susceptibility 

 in the whole head and that the preganglionic region arises as a 

 secondary gradient in the opposite direction from that of the rest of 

 the body. Apparently similar conditions exist during develop- 

 ment in at least many other animal groups, even the vertebrates, 

 the most susceptible region during all except perhaps the earliest 

 stage being not the extreme anterior tip of the head, but a some- 

 what posterior level. Such secondary gradients arise during the 

 course of development and may even disappear later, but it is 

 difficult to determine with certainty how far later changes in 

 susceptibility to particular agents are due to quantitative differ- 

 ences in physiological condition and how far to specific rela- 

 tions. Only when it has been found that, as in earlier stages, 

 the susceptibility relations are essentially the same for a given 

 range of concentration or intensity of many different agents is it 

 reasonably certain that the relations are primarily quantitative 

 rather than specific. 



DIFFERENTIAL ACCLIMATION AND RECOVERY IN HEAD 

 DEVELOPMENT 



Under conditions which permit acclimation or recovery to 

 occur, we find indications of differential effect similar to those 

 already observed in other forms (Child, '16 b, '17; Bellamy, 

 '19); that is, the changes are in the opposite direction from 

 those involved in differential inhibition. Some of these changes 

 observed ar« briefly described. 



Differential acclimation in normal heads. When intact animals ' 

 with normal heads (fig. 17) are placed in low concentrations of 

 various anesthetics, e.g., ethyl alcohol 1.25 to 1.5 per cent; ethyl 

 ether 0.2 to 0.4 per cent, the solutions being renewed every two 

 two days or oftener, the whole head decreases in size relatively 

 to the body, but the preganglionic region undergoes more or less 

 complete reduction, either by resorption or disintegration, until 

 after sixteen to twenty days the head resembles figure 18. About 

 this time, however, or soon after, new growth begins in the median 



