LARVAE OF ARBACIA PUNCTULATA 355 



In solution c, Experiment 3, we have a medium that, if used 

 continuously, would produce maximum growth; but in this ex- 

 periment it has become, at the close of the fifth day, concen- 

 trated to a strength which if employed for the entire period, 

 would have caused a decided decrease in size. At the close of 

 the experiment the larvae are slightly smaller than the control. 

 Although this decrease is almost negligible, it is evident that 

 early exposure has not been so beneficial to the organism as to 

 produce an ultimate increase in size. This becomes the more 

 significant when we consider that the latter period is of shorter 

 duration, — two days as compared with three days of more fa- 

 vorable conditions. In solution d, also, at first the dilution is 

 greater than is most advantageous for development; at the close 

 of 24 hours, evaporation has brought the solution to within the 

 range of the optimum, yet the one day of unfavorable medium 

 reduces the percentage of excess of size to a figure considerably 

 closer to the normal. It seems conclusive, then, that later con- 

 ditions of an adverse nature, although briefly employed, may 

 undo the effects of an early beneficial medium; but this later 

 force has not been able to injure the organism to such an extent 

 that it approaches on either side the limits of growth. 



One further question raised by Experiment 2 remains to be 

 considered.. Is a size greater than the normal indicative of 

 more advanced development, or is it really beneficial to the 

 organism? Resorption of the skeleton at this period of the 

 life history of xA.rbacia is an abnormal process due to inanition 

 of the larvae raised under these experimental conditions, and 

 hence would probably be incurred first by those which have 

 been most vigorous and have developed most rapidly. If rate 

 of resorption of the skeleton be taken then, as an index of de- 

 velopment, it follows from the facts shown in Plot 3 that the 

 individuals of those cultures which have produced increase of size 

 above the control, are not really in a more advanced stage of 

 development. Thus, although the specimens of d and e attain 

 a greater maximum than those of a, they reach it later, indicat- 

 ing an inhibition of development rather than an acceleration. 

 In other words, we cannot conclude that any condition which 



