82 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 
and die in an earlier larval stage the longer they have been 
exposed to lack of oxygen (or to KCN). Eggs which remained 
in the cyanide sea-water only two or three hours after mem- 
brane formation produced plutei in relatively greater numbers 
than those that remained longer in this solution (see chap. iii). 
The optimum length of exposure of the eggs to the potassium 
cyanide solution depends also upon the concentration of KCN 
init. Ina mixture of 50 c.c. of sea-water+2 c.c. 1/20 of 1 per 
cent KCN, three hours seem to represent the optimum length. 
of exposure. If the eggs remain only two hours in the cyanide 
solution, larvae are usually, but not always, obtained. Thus, 
in one experiment the eggs were placed two hours after arti- 
ficial membrane formation in 50 c.c. of sea-water+2 c.c. of 1/20 
of 1 per cent KCN. Of the eggs which were removed from this 
solution after two hours, washed twice in sea-water, and then 
transferred to normal sea-water, 50 per cent developed into 
good larvae; 90 per cent of those removed after three hours 
grew into larvae; but the length of life of the former was greater 
than that of the latter. 
The method of treating the eggs after membrane formation 
with lack of oxygen or with KCN has, however, the disad- 
vantage that it is technically very difficult and, secondly, 
that the time that must elapse between artificial membrane 
formation and transference to the potassium cyanide solution 
is obviously not always the same. I have always found that 
if the eggs are transferred to the cyanide solution immediately 
after membrane formation, none, or only a few, of the eggs 
develop; but I am unable to say how long after membrane 
formation one ought to wait before placing the eggs in the 
cyanide solution. 
These experiments prove at least that it is possible to sub- 
stitute for the corrective influence of a short treatment with a 
hypertonic solution a suppression of the development of the egg 
for a longer period of time. After such a treatment the egg 
