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. 2| 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 a still 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 



