DEDIFFERENTIATION IN ECHINUS LARV.E. 223 



There was, however, very little difference between the effect of 

 Nos. 4 and 5. That is to say, the maximum effect possible with 

 a solution as weak as n/i, 500,000 is, with the amount of tissue 

 used in the experiment, attained with a total bulk of about 100 

 c.c. 



In any experiments concerning the effects of salts of the heavy 

 metals upon living cells, it will therefore be necessary in every case 

 to consider not only the strength but also the amount of the solu- 

 tion, and further the amount of living tissue on which experiments 

 are being carried out. Failing this, the results obtained will not 

 be comparable. However, since in the experi vents on plutei here 

 recorded, the bulk of HgCl, solution was large in comparison to 

 the bulk of the plutei, and since the results are only qualitative, 

 they are not invalidated by the conclusions just reached. 



6. DISCUSSION. 



A. Maintenance of Form. 



Recent work is coming to show more and more clearly how 

 organic form is the product of an equilibrium between constitu- 

 tion (internal forces) and environment. This applies equally well 

 to whole organisms or to their parts. In order that the typical 

 form may be maintained, a particular complex of environmental 

 stimuli is necessary. 



In the case of Ascidians such as Clavellina (Driesch, '06; 

 Schultz, '07) and Perophora (Huxley, '21 b) whole zooids below a 

 certain size when exposed to unfavorable agencies, such as ac- 

 cumulated waste products or dilute solutions of KCN, are unable 

 to maintain their form, and undergo what is known as dedifferen- 

 tiation. 



The same is true of Hydroid polyps (Loeb, 'oo). In Hydroid 

 polyps we (Huxley & De Beer, in the press) have succeeded in 

 finding a quantitative relation between the rapidity of the form- 

 changes and the concentration of the toxic solutions employed ; 

 this is also seen in the present study (Table I.). 



The encystment of Protozoa in times of drought or cold is an 

 example of the same phenomenon. 



The remarkable dedifferentiation of the hydriform larva of 



