396 Ralph S. Lillie 



resulted. Another portion of the same eggs was similarly treated 

 in the afternoon about three hours after completion of maturation; 

 a large proportion failed to form membranes, cleavage was either 

 irregular or failed altogether, and not a single larva resulted. 

 This series was less favorable than those tabulated above, and the 

 optimum exposures were considerably more prolonged. The dif- 

 ference in the time of year may be a factor of importance; this, 

 however, can only be determined by further experiment. On the 

 whole, when normal eggs are used a given temperature has a 

 quite well defined optimum period of exposure which can be de- 

 termined with considerable accuracy. Since the temperature- 

 coefficient of acceleration of a given process may afford valuable 

 indications as to its essential nature, a more exact redetermi- 

 nation of the optimal periods of exposure through a greater 

 range of temperatures may yield valuable results. I hope at 

 some future time to make more extended and exact determina- 

 tions of the above and similar relations. 



Susceptibility to Warming at Different Periods During Maturation 



The foregoing experiments had shown that momentary warming 

 has a favorable action in inciting parthenogenetic development 

 during, but not after, the period of maturation. It remained to 

 determine more precisely the limits of the period of susceptibility 

 to this form of treatment, and the variation in favorability within 

 this period. 



For this purpose in each of the series of experiments tabulated 

 in Table IV the eggs of a single female were employed; successive 

 portions of these were warmed to 35° for 70 seconds, beginning 

 about five minutes after removal (at a time when the germinal 

 vesicle had undergone no visible alteration), and thereafter at 

 regular five minute intervals until the separation of the first polar 

 body had taken place. The condition of the eggs at the time 

 of warming was observed in each case in a "control" portion 

 kept under the microscope throughout the entire period of the 

 series. With good eggs from a single female the maturation proc- 

 ess progresses with almost uniform velocity in all eggs; the numer- 

 ous eggs of each portion may thus be considered practically uni- 



