THE PHYSIOLOGY OF DEVELOPMENT 205 



they will, according to Child's interpretation, inhibit regions 

 of higher activity to a more marked degree than regions of 

 lower activity ; while, on the other hand, in very low con- 

 centrations of the reagent such as to permit acclimation 

 and recovery, the region of higher activity will be inhibited, 

 according to Child's interpretation, less than regions of lower 

 activity. 



Two instances of the use of the susceptibility method 

 must suffice to indicate some positive results of the application 

 of Child's hypothesis. Eggs of Polychaetes were placed in 

 lethal concentrations of KCN. Initially dissolution begins at 

 the anterior end. As development proceeds the region of 

 maximum susceptibility shifts to the posterior region (where 

 growth is most active) so that when the larva is ready to undergo 

 metamorphosis the posterior extremity is the region which 

 succumbs most readily to lethal and recovers most easily 

 from sublethal concentrations. Child (19 17) finds that 

 embryos submitted to short exposure of lethal concentration 

 in the earliest developmental stages develop into individuals 

 with abnormally small heads. Embryos which are similarly 

 exposed at the later stage develop into individuals with 

 abnormally large heads. 



In a similar study by Child of development in sea-urchins, 

 long exposure to sublethal concentration as well as the short 

 exposure to lethal concentration was investigated. Two 

 resultant types of plutei are figured, that produced by " dif- 

 ferential acclimation " (long exposure) with abnormally 

 enlarged oral lobe and widely divergent arms and that produced 

 by " differential inhibition " (short exposure) with diminished 

 oral lobe and angle of divergence between the arms. It is 

 impossible in the short space at our disposal to do justice to 

 Child's voluminous pubHcations, which must be consulted 

 for further information. The susceptibility method may well 

 prove a useful instrument of research, when its theoretical 

 assumptions are independently substantiated by accurate 

 gas analysis. Shearer (1924) in a recent publication records 

 results of an investigation based on direct measurement of 

 oxygen consumption with a technique that for the purpose is 



