LARVAL DEVELOPMENT IN THE SEA URCHIN 97 



HCN m/200000 the early stages of the plutei may show a slightly 

 inhibited oral lobe, but later stages are usually normal in form. 

 Development in concentrations higher than m/50000 usually 

 ceases at the blastula stage and death occurs, the apical region 

 dying first, and concentrations below m/200000 have no appre- 

 ciable effect on development except, perhaps, a slight retardation. 



In recovery after the temporary action of the higher concen- 

 trations of cyanide there are, of course, many cases in which the 

 more apical regions are killed and only a larger or smaller part 

 of the basal region recovers. Such cases may undergo more or 

 less reconstitution, according to the stage of development, and 

 may produce plutei. The following data give some idea of 

 concentrations and periods of action. 



Unfertilized eggs, 3 hours in KCN m/1000, then washed and fer- 

 tilized. Practically all develop normally and produce normal 

 plutei. Any differential inhibition which may have occurred in 

 earlier stages is comepensated by differential recovery. 



Unfertilized eggs 6| hours in KCN m/1000, then washed and 

 fertilized, 60 to 70 per cent develop, many with abnormal cleav- 

 ages. In nearly all a larger or smaller portion of the apical region 

 dies and is lost before the gastrula stage is reached; i.e., such 

 portions have been so far inhibited that they are incapable of 

 recovery. 



The partial forms undergo more or less reconstitution and the 

 resulting forms show all degrees of differential inhibition with 

 general and some degree of differential recovery superimposed 

 (figs. 6, 57 to 73, 82 to 90) and, as the extreme type, spherical 

 basal forms without reconstitution (figs. 19 to 21). 



Unfertilized eggs 8^ hours in KCN m/1000, then washed and 

 fertilized: 40 to 50 per cent show some development, but 20 to 

 30 per cent of these die before the gastrula stage, and more or 

 less loss of the apical regions occurs in others. The forms pro- 

 duced show the same range of variety as in the preceding lot, 

 but the percentage of the more extreme types of differential 

 inhibition with less differential recovery is greater. 



In these cases of inhibiting action of KCN before fertiliza- 

 tion, the period of recovery is the whole period of development, 



JOURNAL OF MORPHOLOGY, VOL 28, NO. I 



