NOTES ON VARIATION IN DURATION OF SIMILAR 



PERIODS OF EMBRYONIC DEVELOPMENT: 



ITS BEARING ON THE THEORY OF 



EFFECTIVE TEMPERATURES. 



By A. A. Girault. 



Perhaps attention should be called to the fact, which, however, 

 is obvious, that in determining the relations of periods of develop- 

 ment to effective temperatures one of the factors involved is often 

 obtained approximately only ; or in other words is measured with- 

 out any reference to its variation. This factor, of course, is the 

 period of development. Very little has been written concerning 

 certain phenomona exhibited by these periods. For instance, in 

 determining the periods of embryonic development — the egg 

 stage — is it necessary, for the purpose in view, for any single 

 record to be made with eggs deposited by a single parent ? Would 

 several parents of these eggs affect the results, providing the eggs 

 were all deposited at the same time and kept together? Do two 

 eggs deposited simultaneously (i. e. in quick succession) by the 

 same parent and kept in the same environment hatch at the same 

 time? If not, how is- the period recorded? If not, how would the 

 period be recorded for 50 eggs deposited in the same manner by 

 a single parent? If the period of embryonic development is 

 variable in the case of two eggs deposited simultaneously by the 

 same or different parents or for any number of eggs deposited 

 in rapid succession, how is it to be measured? Does not this 

 mean that the probable error will have to be determined first from 

 a large number of simultaneous observations? Or is the case 

 such that this factor may be ignored entirely ? The latter is most 

 probably true — since the duration of the period of hatching for 

 eggs of the same or different parentage deposited simultaneously 

 and kept in similar environments are limited to short periods of 

 time — as far as we know — and therefore the effective temperature 

 recorded for either limit would vary not at all or very imper- 

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