Artificial Parthenogenesis in Starfish Eggs 395 



The different temperatures do not however seem equally favor- 

 able, and more larvae are obtained at 35° and 36° than at 37° and 

 38°; in other words, with the higher temperatures warming seems 

 more likely to produce abnormal results. My experience has been 

 that temperatures of 35°, acting for somewhat longer than one 

 minute, have usually given the best results. This is illustrated by 

 the foregoing tables and again by the following four experiments. 

 In this series eggs from a single animal were exposed to 35° for 

 60, 70, 80 and 90 seconds, respectively, at a period of 30 to 35 

 minutes after removal from the animal. The results were as 

 follows : 



An exposure of 70 seconds to 35° again proves most favorable. 

 In all of the following experiments on the determination of the 

 period of greatest susceptibility to this treatment I have accordingly 

 employed uniformly an exposure to 35° for 70 seconds; such treat- 

 ment if applied at a favorable period (best at some little time before 

 the separation of the first polar body) almost invariably results in 

 the production of a good and sometimes a high proportion of 

 larvae. There appears however to be some variation in the opti- 

 mum period of exposure to a given temperature in eggs from differ- 

 ent animals and at different seasons of the year. Thus on Septem- 

 ber 6, 1906, eggs were treated as follows during early maturation 

 (17 to 37 minutes after removal) : 35° for 70, 75, 80 and 85 seconds; 

 36° for 40, 45, 50, 55 and 60 seconds; 37° for 20, 25, 30 and 35 

 seconds. Cleavage was most nearly normal and a certain rather 

 small proportion of larvae was obtained with 35° for 85 seconds, 36° 

 for 50, 55 and 60 seconds (the last best), and 37° for 30 and 35 

 seconds (both about equally good). With the other exposures the 

 cleavage was slower and less regular and no swimming larvae 



