DEDIFFERENTIATION IN ECHINUS LARV.E. 23! 



that toxic agencies are important in the initiation of metamorphosis. 

 Cytolysing agencies are not necessary to explain resorptive de- 

 dififerentiation such as occurs in Hydroids and Perophora; they 

 should therefore not be postulated, as is done tentatively by Runn- 

 strom, to explain metamorphic changes in Echinoderms until 

 definite proof can be adduced of their existence. 



A further corroboration of the idea of metamorphosis as an 

 upset of balance is to be seen in Runnstrom's own remarkable 

 observations upon the formation of pedicellarise. In much-re- 

 duced larvae, more or less well-differentiated rudiments of pedicel- 

 larise often appear ab initio. Once pedicellariae have been formed, 

 however, they are among the first organs to be dedifferentiated. 

 This may be interpreted by supposing that the potency of produc- 

 ing pedicellariae is present in larvse, but normally inhibited until a 

 certain stage by the demands of the existing organs. When these 

 are reduced, however, the check is removed and the pedicellaria 

 develops. Once developed, on the other hand, its cells are not 

 actively growing, and are easily affected by unfavorable agencies. 

 This is very similar, it would seem, to the behavior of larval and 

 adult organs at metamorphosis. 



With Runnstrom's general discussion I find myself in full 

 agreement in all essentials, except that I do not consider that he 

 has laid sufficient stress upon the direct inhibiting effect of poisons 

 upon growing tissues, an effect which leads eventually to dedif- 

 ferentiation by a different route from that consequent upon 

 hunger. 



APRIL 6, 1921. 



SUMMARY. 



1. The retardation of early Echinus development caused by 

 toxic agents (KCN and HgCU) is noted. 



2. When recovery took place on transference to sea-water after 

 48 hours in the toxic solutions (i.e., in the blastula stage), plutei 

 were formed. These were mostly very wide-angled ; this was 

 due to the greater power of recovery of the oral region. 



3. On transference to sea-water after 72 hours' treatment with 

 the poisons (i.e., in the gastrula stage), development only took 

 place as far as the pre-pluteus stage. Considerable power of re- 



