PHASES OF DIVIDING SEA-URCHIN EGG. 133 



ethyl and capryl show a higher toxicity than the others. 1 It 

 may be that differences, both qualitative and quantitative 

 in the lipoid elements of the tissues, and hence in the plasmamem- 

 brane, form the basis of the observed physiological difference. 



In the controls which were running parallel to the two exper- 

 iments just described, a majority of the eggs entered the two- 

 celled stage at about fifty-eight minutes after fertilization. At 

 sixty-five minutes after fertilization, between 85 and 90 per cent, 

 had cleaved. Practically all eggs in the controls had reached the 

 blastula stage the following day. 



HEPTYL ALCOHOL. 



A series of six experiments were performed with this alcohol 

 to determine the limits of suitable concentration. Reasoning 

 from the data in the proceeding, it was thought that the optimum 

 concentration would be in the proximity of between 0.04 and 

 0.07 vol. per cent., but to make sure, concentrations as low as 

 0.02 and as high as 0.08 vol. per cent, were used. As a matter 

 of fact, solutions of 0.06 and 0.07 vol. per cent, were found to be 

 the best suited to the experiments, although it is interesting to 

 note that even weaker solutions showed marked toxic action on 

 eggs at the time of formation of the first cleavage furrow, while 

 during the stages preceding and succeeding division, they had 

 relatively little influence. Table III. summarizes three exper- 

 iments with this alcohol, and is fairly typical of results obtained 

 in other experiments. By some unavoidable oversight in tech- 

 nical procedure, records for the first two periods (i.e., respectively 

 10 and 20 minutes after fertilization) were not obtained, but 

 from the results of other series it is evident that the eggs maintain 

 a rather high resistance at these times. The results given in 

 Table III. show that 0.07 vol. per cent, approximates the favor- 

 able concentration for this alcohol, while the solutions on either 

 side are slightly hypo- and hyper- toxic respectively; i.e., in the 

 one case practically all eggs survive to the blastula stage, and in 

 the other nearly all die. Recovery of resistance after the first 

 division appears to be relatively slow. When the percentage of 

 surviving blastulse is plotted against the time intervals, regarding 



2 See ibid., p. 133. 



