PARTHENOGENESIS IN THE SEA URCHIN. 205 



egg-water of 5. franciscanus, but are intensely activated in the 

 actual presence of ova of the latter and form dense halos around 

 them. Such eggs, freed from the excess of sperm by repeated 

 washing in sea-water, fail to develop and those with membranes 

 cytolyze in sea-water within twenty hours. If, however, the 

 washed ova are subjected to a treatment with Loeb's hypertonic 

 sea-water a certain percentage of those with membranes develop 

 into gastrulae and a smaller percentage into plutei. The best 

 results were obtained when the ova were exposed to a concen- 

 trated sperm suspension for one and a half minutes and then to 

 hypertonic sea-water for seventy minutes. At the end of sixty 

 hours there were twenty-eight per cent, gastrulse and seventeen 

 per cent, plutei. The remainder had cytolyzed. The plutei 

 lived for eleven days and the majority were normal in appearance. 

 Similar results were obtained with concentrated sperm suspen- 

 sions of Ishnochiton magdalemis and of Cryptochiton. Ova 

 treated with hypertonic sea-water only for similar periods did 

 not develop and such ova could subsequently be fertilized in 

 sea-water with species-true sperm. 



The blood serum of K. tunicata produced results similar to 

 those obtained with sperm filtrates or with the concentrated 

 sperm suspensions of this species. If concentrated serum is 

 used the resulting membranes are wrinkled and unevenly ele- 

 vated. If the serum is diluted twenty to thirty times normal 

 membranes are produced. (Robertson (1912) obtained his best 

 results with diluted serum in experiments with ox blood on ova 

 of 5. purpuratus.) The subsequent cytolysis in sea-water can 

 be prevented and the development of many of those with mem- 

 branes obtained if the ova are treated with hypertonic sea-water 

 for seventy minutes after exposure to the serum. Similar re- 

 sults were obtained with dilutions of the serum of Ishnochiton 

 magdalensis and of Cryptochiton. 



The ova of .S. purpuratus were used in similar series of experi- 

 ments. In place of a typical membrane, ova exposed to test 

 preparations, appeared to be surrounded by a swollen transparent 

 jelly which tended to dissolve in sea-water. Such eggs could be 

 fertilized in sea-water with sperm of the same species at once 

 or two hours later. Such eggs if treated with hypertonic sea- 

 water did not form membranes or develop. 



