LARVAL DEVELOPMENT IN THE SEA URCHIN 99 



concentrations of KCN may be used on blastula or gastrula 

 stages, e.g., ra/100, 2 to 2\ hours, m/200, 4 to 4^ hours. With 

 such concentrations and times total death occurs in 75 to 90 

 per cent, and a greater or less degree of apical partial death in 

 the remainder. The partial basal forms which remain alive 

 undergo more or less reconstitution, according as the living basal 

 portion represents more or less of the apico-basal axis. The re- 

 sulting forms show mostly the more extreme degrees of differ- 

 ential inhibition. A small percentage of narrow angle plutei 

 (fig. 6) or forms with parallel or fused arms occur (figs. 82 to 

 87), also some forms with apical reconstitution (figs. 57 to 81), 

 but 50 per cent or more of those which remain alive never de- 

 velop beyond the spherical stage (figs. 19 to 21). 



It is evident from these data that a wide range of concentra- 

 tions of KCN may be used without altering the results very 

 greatly, except as regards the percentages of deaths and of dif- 

 ferent forms. Within certain limits the chief difference is 

 that, the higher the concentration, the larger the percentage of 

 total deaths and partial deaths and so the smaller the percent- 

 age of living forms and the larger the percentage of partial forms 

 which show no differential recovery or only apical reconstitu- 

 tion. Where the whole development occurs in KCN, differen- 

 tial acclimation is very slight, and occurs in very few individuals. 



Ammonium hydrate 



In the case of NH 4 OH much higher concentrations than of 

 KCN must be used to produce differential axial effects on de- 

 velopment, but in concentrations high enough to produce such 

 effects, differential inhibitions with no appreciable differential 

 acclimation result. Eggs developing in NH 4 OH m/10000 from 

 the first cleavage produce practically normal forms, the only 

 indication of differential inhibition being a slightly reduced oral 

 lobe. 



Eggs developing in NH 4 OH ?ft/5000 from first cleavage, show 

 25 to 50 per cent of total deaths and most of the remainder show 

 partial apical death. 80 to 90 per cent are spherical basal 

 forms like figures 19 to 21, without reconstitution or differential 



