96 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 
not all form normal larvae. Some of the eggs have remained 
too long in the hypertonic solution and this over-exposure 
injures the eggs concerned. This is proved by the fact that the 
number of injured larvae is always the greater the later the eggs 
were taken out of the hypertonic solution. What determines 
this individual variation we cannot say. In part it is connected 
with the unequal distribution of the eggs in the vessel, whereby 
the access of oxygen to some eggs is more complete than to 
others. This is, however, only one of several factors. It is an 
important fact that even a slight over-exposure to the hyper- 
tonic solution injures the eggs. 
How long ought the eggs to remain in the hypertonic solu- 
tion? This again depends upon how long after membrane 
formation they are placed in the hypertonic solution. A. R. 
Moore has carried out some experiments on this subject in my 
laboratory on S. purpuratus. He determined the time which is 
necessary to cause all the eggs to develop into larvae. When 
the eggs are placed in the hypertonic sea-water (50 c.c. of 
sea-water+8 c.c. 23 m NaCl) immediately, i.e., two to four 
minutes after membrane formation, they must remain sixty 
to seventy minutes in the hypertonic solution before all can 
be caused to develop. When placed in the hypertonic solution 
thirty minutes after membrane formation, they must remain in 
it forty to fifty minutes in order to obtain the best results. If 
transferred to the hypertonic solution one or two hours after 
artificial membrane formation, it was only necessary to leave 
them in it thirty to forty minutes in order to get all the eggs to 
develop into larvae. If astill longer interval elapses before the 
eggs are put in the hypertonic sea-water after membrane forma- 
tion, the results again become worse. In these experiments the 
temperature was about 12° C. 
In order to give the reader a more perfect idea of what has 
been said I will describe more completely one of my own obser- 
vations. Membrane formation was produced in the eggs by 
