262 PATTERNS AND PROBLEMS OF DEVELOPMENT 



blastopore, giving various degrees of spina bifida, usually with micro- 

 cephaly, sometimes almost complete acephaly. In cases of spina bifida two 

 tails often develop, usually more or less bent dorsally, indicating greater 

 inhibition on the more susceptible dorsal side (Fig. io6, D). This dorsal 

 curvature of the tail or tails is highly characteristic of differential inhibi- 

 tion, even in absence of spina bifida (Fig. io6, E). All degrees of medio- 

 lateral differential inhibition appear in the head region. Ventral suckers, 

 olfactory pits, and eyes develop with increasing inhibition progressively 

 nearer the median plane, may become single median organs, often with 

 decrease in size, or may be entirely absent; and inhibited forms almost or 

 quite acephalic occur. Since different regions of the embryo attain high 

 susceptibility at different developmental stages, the forms resulting from 

 differential inhibition differ according to stage and period of exposure to 

 the agent. For example, with temporary exposure in earlier stages the 

 head region may be greatly inhibited, the tail almost or quite normal, 

 because the tail region was not highly susceptible at the stage of exposure. 

 With temporary exposure at certain later stages the head may be little, 

 the tail greatly, inhibited; but these differences apparently do not repre- 

 sent specific differentiations — at least they are not specific for particular 

 agents. 



The most conspicuous secondary modification with the less extreme 

 degrees of inhibition in early stages is development of a relatively large 

 tail bud with ventral curvature of the developing tail (Fig. io6, F), that 

 is, greater elongation of the dorsal side, a modification opposite in 

 character to that of direct differential inhibition (Fig. io6, D, E). These 

 forms are frequent with return to water after temporary exposure and 

 appear, beyond question, to be cases of dorsiventral differential recovery. 

 They are less frequent with continuous exposure, but sometimes appear 

 apparently with differential conditioning, though less extreme than after 

 return to water. Animals inhibited to the spina bifida condition (Fig. io6, 

 A-D) usually show little or no secondary modification; they may undergo 

 some further development after return to water, but differential inhibi- 

 tion remains predominant. 



With low concentrations of NaOH and of alkaline KCN and by addi- 

 tion of HCl to a well water with high carbonate content Bellamy (1922) 

 obtained acceleration of development with indications of differential ac- 

 celeration in precocious head development and megacephalic tadpoles 

 (also Child, unpublished). 



Forms described by Higgins and Sheard (1926), resulting from daily 



