DEDIFFERENTIATION IN ECHINUS LARWE. 



of axial gradients, the susceptibility of cells to toxic agents, and 

 their readiness to dedifferentiate, are in part functions of their 

 surface area. 



D. Previous References to De 'differentiation in Echinoid 



Larvce. 



Yernon ('94) found that various environmental factors had a 

 marked effect on the arm-length of plutei (the experiments wen* 

 continued up to the 8-day stage). The effect, whether caused by 

 actual resorption of tissue present, or failure to grow beyond a 

 certain length, means that equilibrium with these slightly unfavor- 

 able conditions could only be maintained by arms of a shorter 

 length than normal. 



Robertson ('13) found that Strongylocentrotus blastulse trans- 

 ferred to a medium containing 0.15 per cent, lecithin for 24 hours 

 and then returned to sea-water, not only were very much retarded 

 in development, but after becoming gastrulse nearly lost their gut 

 again before dying (figures and details are not given). He 

 ascribes this result to a specific action of lecithin on growth-proc- 

 esses. From what we know of Echinoderm development, the 

 result, in the absence of further data, might at least equally well 

 be non-specific and due to dedifferentiation. 



In addition, the literature abounds with notices that " un- 

 healthy ' larvse may show shortened arms, sometimes with a 

 terminal protrusion of spicules. (And see postscript, p. 230.) 



E. Recovery. 



The meager results on recovery merit one or two words. Fu- 

 ture experiment must decide between one or two possibilities. 

 Either the Hg + ion damages only a fraction of the cells' proto- 

 plasm, which, if it does not exceed a certain critical value, may 

 be repaired, and normal differentiation resumed ; or repair is not 

 possible, and the animal cannot differentiate further. The theo- 

 retical bearings of this second possibility have been discussed by 

 Child ('16) and I do not intend to go into them at present. What 

 is interesting in either eventuality is the extremely long period 

 for which the dedifferentiated organisms can remain not merely 



