APPENDIX VIII 



Most of the experimental modifications of echinoderm development recorded else- 

 where have resulted from action of single or a few concentrations or intensities of action 

 of external agents. Many are effects of artificial sea waters differing from natural sea 

 water in absence or increase in amount of certain components or substitution of 

 certain components by others not present or present only in very small amounts in 

 natural sea water. In spite of the great volume of experiment on echinoderm develop- 

 ment there have been relatively few attempts to determine for single agents the effects 

 of a wide range of concentrations, intensities, and exposure periods. It seems often to 

 have been taken for granted that the modification produced by a particular concen- 

 tration or intensity is specific. Also, the possibility of secondary modifications resulting 

 from differential tolerance, conditioning, or recovery, and opposite in direction as re- 

 gards form and proportions from primary differential inhibitions, has been largely 

 ignored. In view of this situation a few further data concerning effective ranges of 

 concentrations, exposure periods, etc., supplementing those in the figure legends, are 

 given here. 



With any agent in any effective concentration or intensity a considerable range of 

 modifications occurs. These depend in part on differences in susceptibility of individual 

 eggs and eggs of different females, but difference in conditions to which individuals are 

 subjected at different stages of development in the container may also be a factor. 

 Animals remaining on the bottom instead of swimming, and particularly aggregations, 

 if allowed to persist, show more extreme differential inhibition with certain agents 

 (e.g., KCN, LiCl) than animals swimming free. On the other hand, concentration of 

 an inhibiting agent may become lower about an aggregation of animals because the 

 agent is taken up by the cells. If there is a free surface exposed to air, animals reaching 

 that surface may be much less inhibited than those that do not reach it. These effects 

 of local differences in conditions in the container can be decreased by frequent gentle 

 agitation, by decreasing surface exposed to air, and by use of large volume of medium. 

 Undoubtedly, very slight physiological or environmental differences may determine 

 whether the final result in a particular individual is a modification representing differ- 

 ential inhibition or one with some degree of differential tolerance, conditioning, or re- 

 covery. It is true, however, that differential inhibition is universal or predominant with 

 higher concentrations or intensities and longer exposure periods; with decrease in con- 

 centration differential tolerance and conditioning become increasingly evident; and 

 with certain ranges of concentration or intensity the modifications of differential re- 

 covery become predominant following return to water. Degrees of secondary modifica- 

 tion differ widely with different agents: with KCN, for example, differential tolerance 

 or conditioning is very slight or absent in the brief period of development, but more or 

 less differential recovery may occur after return to water from low concentrations. With 

 LiCl the secondary modifications occur to a much greater degree; with alcohol and 

 various other agents they occur rapidly and to extreme degrees. The differential in 



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