237 
4 
_ various periods (column 3) in order to get the total accumulation of day- 
degrees. These are recorded in column 9. If, in column 9, we disregard 
the accumulations for the first two groups at the top of the table, the 
accumulations are fairly constant till the 78-79 group is reached. Below 
this the accumulations rapidly increase, owing to the fact that the aver- 
_ ages in column 6 contain some temperatures that are above the degree 
of the maximum rate of development and so retard development. It 
_ was necessary, therefore, to ascertain the degree of maximum rate of 
_ development, and to make the necessary corrections in the daily day- 
degrees column, so as to get only the effective day-degrees. In deter- 
“mining this point and making the correction, it was assumed that the 
products obtained by multiplying the average daily effective day-degrees 
by the periods would be nearly the same for all temperatures. It will 
' not be necessary-to give the details of the process by which the degree 
_ of maximum rate of development and the effects of temperatures above 
_ this point were determined. Suffice it to say that they were found by 
making guesses and testing the guesses out. It was found in this way 
_ that if 88 degrees were taken as the degree of maximum rate of develop- 
ment and twice* the number of day-degrees above this point (column 7) 
_ were subtracted from the average day-degrees above 50 degree, values 
_ for the daily effective day-degrees (column 8) were secured which when 
_ multiplied by the periods gave products that very nearly filled the condi- 
tions, giving about the same accumulation of effective day-degrees for 
_ the high temperatures as for the low. 
; By plotting the average daily day-degrees obtained in column 8 
against the reciprocals of the periods, the points all fall somewhere near 
the established line (Graph 2). The average accumulation of effective 
_ day-degrees for the incubation period is, therefore, about 163. 
__ No good reason can be assigned why the accumulations for the 60-61 
and the 62-63 groups should be higher than the average unless it may 
have been that in assuming in 1915 that eggs found in the forenoon of 
one day were laid the previous evening, too long a period was assigned 
to many of them. The weather during the early part of 1915 was very 
_ cloudy and a larger per cent. of the eggs were laid during the daytime 
than in the other seasons. 
U 
, 
: ’ * Twice the number of day-degrees above the degree of maximum rate of development 
» is subtracted from the total day-degrees above the zero of development because one degree 
_ rise in temperature above the degree of maximum rate of development retards the rate 
of development at approximately the same rate as does one degree fall in temperature 
below this point. 
