20 Bulletin Wisconsin Natural History Society. [Vol. 8, No. 1'. 



periods of deposition and hatching, calculating the result from 

 the mean times of the two periods. Hence the variation described 

 in foregoing apparently has little or no effect on the present 

 methods of measuring periods of embryonic development ; but it 

 is also apparent that as yet we do not know any too much con- 

 cerning it. We have, in fact, neglected to observe it at all. 



Its bearing on the theory of effective temperatures is no more 

 than this : If it proves to be very large and is left out of con- 

 sideration it is obvious that true relations will not be obtained; 

 it is also obvious that true relations may not be obtained if it 

 proves to be very small and is neglected. It is best therefore, to 

 determine it in all cases, if for no other purpose than to know to 

 what extent it occurs. It is important also from the standpoint 

 of physiological variation, for is not that kind of variation as 

 important as morphological variation which we hear so much 

 about ? 



Urbana, 111., 1909. 



