428 GRACE MEDES 



from the above data. The following table gives the increase 

 in size in percentages above the average of the control in each 

 of the foregoing experiments. (Values are not given where 

 decreased size has been produced nor after the control has 

 passed the highest point on its growth^curve.) 



This table gives some idea of the degree to which size may 

 be increased in Arbacia larvae under experimental conditions. 

 The percentage may be comparatively large when the embryos 

 are 24 hours old, but becomes gradually less until finally when 

 the larva are 120 hours old, it is hmited to 4.6 (Experiment 14). 

 This represents approximately the maximum increase in size, 

 since if the factors which produce this excessive growth are 

 applied in greater concentrations, an inhibition rather than a fur- 

 ther increase occurs. But these figures do not stand for absolute 

 values, since the controls, themselves, represent varying quanti- 

 ties. Would the effect be greater if the values of the control 

 were low? ^The first experiment with dilutions (1) seems to in- 

 dicate that it would, in the early stages; but at the close of the 

 experiment increase of size is not relatively greater than in the 

 other experiments. 



Decrease in size may be so great under experimental condi- 

 tions, that some cultures never approach the range of the con- 

 trol, — for example, the specimens in the second experiment 

 in which NaCl is added to the sea-water, do not acquire skele- 

 tons until 96 hours old. These forms resemble the 'Dauer- 

 blastulae' of Driesch, but have archenterons fully developed or 

 short and bent over so that the mouth opens close to the blasto- 

 pore. Strictly speaking, however, they are not Dauer-blastulae 

 for if subjected to a more favorable medium they may resume 

 development, but may not, as we have seen, '' attain the maxi- 

 mum size to which they can develop." On the other hand, 

 the larvae may endure adverse conditions of greater intensity 

 if subjected gradually to the medium than if subjected sud- 

 denly, as shown by the experiment with gradually concentrated 

 sea- water (3). 



The excessively low values which may be produced in the 

 early stages, indicate so great an injury that life cannot long 



