STUDIES IN ARTIFICIAL PARTHENOGENESIS. 155 



fashion that the volume enclosed within it might be lessened. 

 Whereas an extremely rigid membrane would resist such forces, 

 one with only a certain degree of rigidity would yield (in the case 

 of sufficient pressure), and exosmosis would be possible. Thus 

 osmosis in an enclosed system depends, to some extent at least, 

 on the rigidity of the confining membrane. These conclusions 

 apply in some measure to the sea-urchin egg, for the vitelline 

 membrane possesses a slight degree of rigidity. Most salts in 

 hypertonic solution cause the membrane to absorb water and 

 swell, the gel becomes less stiff, and the particles of the mem- 

 brane can more readily rearrange themselves. Thus shrinkage 

 of the egg is favored. We should in fact expect that hyper- 

 tonic solutions of salts which cause membrane swelling would 

 be more effective in causing shrinkage than those which do not. 

 Direct evidence of this fact is difficult to obtain without in- 

 troducing complications. One might compare the shrinkage of 

 the egg in two solutions of equal osmotic strength, one of 

 which causes membrane swelling and the other of which does 

 not. But we have no means of knowing when two solutions 

 are of equal osmotic power. Vant Hoff's law does not apply 

 with sufficient accuracy to warrant its use, 1 and even if two 

 solutions could be obtained which were isosmotic towards a 

 given membrane, they would not necessarily be isosmotic 

 towards the plasma-membrane of the Arbacia egg, which is very 

 probably only partially semipermeable. Fortunately theie is a 

 way out of the difficulty. I found that when 2.5 N NaCl was 

 added to sea-water in the proportion of 8 parts by volume of 

 the former, to 50 of the latter, the resulting hypertonic solution 

 would usually cause membrane swelling when freshly prepared, 

 but would in large measure lose this power after it had stood for 

 some time. Thus one can obtain two solutions of identical 

 strength, one of which produces a softening effect on the mem- 

 brane, the other lacking this effect. A number of experiments 

 showed that in every case the eggs shrank more in the solution 

 which caused membrane swelling than in the solution which 

 left the membrane with its original rigidity. Two sample 

 experiments are recorded here. To save time the term "NaCl 



1 Cf. Findlay, "Osmotic Pressure," London, 1913 (Chapt. IV.). 



