98 C. M. CHILD 



and during this period the lesser degrees of differential inhibi- 

 tion may be more or less completely compensated by differential 

 recovery, so that the final form may approach the norm, al- 

 though, in the earlier stages, there is marked differential inhi- 

 bition or even apical loss. Where the degree of inhibition and 

 therefore the apical losses are greater, differential recovery 

 brings about more or less apical reconstitution, but in the most 

 extreme cases which remain alive and develop, there is no recon- 

 stitution, but only a partial basal form is produced. In con- 

 sequence of these various possibilities, the range of variation in 

 form in a single lot of eggs is greater than where the inhibiting 

 agent acts on later stages. 



Eggs placed in KCN m/20000-m/10000 at the beginning of 

 cleavage usually continue to develop slowly, reaching the early 

 blastula stage in 12 to 18 hours, but by the time they reach this 

 stage their susceptibility has increased (Child, '15 b, pp. 412- 

 418) to such an extent that further development is inhibited. 

 If returned to water at this time a considerable percentage of 

 total and of partial apical death occurs, and the final stages 

 show all degrees of differential inhibition, with differential re- 

 covery in the form of apico-basal elongation and .apical re- 

 constitution (figs. 57 to 81) in a varying percentage. The same 

 forms may be produced by a few hours in KCN m/10000 at the 

 blastula stage. In fact, for any concentration which is low 

 enough to permit development to proceed to the blastula stage, 

 the effect is essentially the same, whether the eggs are placed 

 in KCN at early or late cleavage and left there until they reach 

 the blastula stage, or are placed in the same concentration at the 

 blastula slage. 



Late cleavage or early blastula stages in KCN m/10000 never 

 develop beyond the blastula stage. On return to water after 36 

 hours in this concentration, development proceeds with a vary- 

 ing percentage of partial apical death, but few total deaths. 

 Such series give the usual degrees of differential inhibition, with 

 more or less differential recovery in the apical region in 25 to 

 50 per cent, appearing as apical reconstitution in some of the 

 partial forms. With a very short period of action, very high 



