DEDIFFERENTIATION IN ECHINUS LARVAE. 21 7 



4). The pair was isolated in sea-water, but was dead by 120 

 hours. 



The progress of dedifferentiation in HgCl 2 /2,ooo,ooo was 

 similar but considerably slow-er. 



This is shown in Fig. 5, which represents an advanced speci- 

 men after 120 hours (by which time all those in nf 1,000,000 were 

 dead). An abnormal addition to the skeleton is seen. This may 

 perhaps be due to the deposition in the aboral half of calcium 

 carbonate resorbed in the arms, or to the fact of the spicule- 

 secreting tissue being less affected than the rest, as Child ('16; 

 suggests. The mouth was, I think, closed; the oesophagus was 

 swollen. Most individuals after 120 hours were in the same stage 

 of dedifferentiation as shown by the n/ 1,000,000 culture after 48 

 hours. 



Partial death was not seen till 144 hours; complete death had 

 occurred by 168 hours. As in the /i,ooo,ooo culture, the still- 

 living portion when partial death Had set in was very narrow, the 

 spicules being nearly parallel. This narrowness is a result of 

 all regions being affected simultaneously. 



The controls for this experiment remained very healthy and 

 off the bottom for 96 hours. The culture was rather crowded 

 and not fed. After 120 hours, most were on the bottom, and 

 were showing a certain amount of arm-resorption (though less 

 than that shown in / 1,000,000 after 24 hours) . One was seen with 

 3 arms reduced to minute knobs, but the fourth perfect. This 

 was never seen in the HgCL cultures, although there the arms of 

 one side might be resorbed faster than those of the other. 



The resorption in the controls might have been due either to 

 starvation, or to the accumulation of toxic products,, or both, but 

 tests were not made to determine the point. Runnstrom's results 

 ('17), show that either alone can cause dedifferentiation. 



All were on the bottom after 144 hours. They remained in 

 this position with almost total absence of arms, but healthy in 

 other respects, for 5-7 days longer. 



F. Nine-day plutei from the main control culture (fed on 

 Nitzchia} were transferred to HgCL /i, 000,000. 



The appearance of the more advanced of these is shown in 

 Fig. 6. All had developed the third pair of arms. 



