50 BULLETIN 903, U. S. DEPARTMENT OF AGRICULTURE. 



Examination of Table VIII shows that the incubation period 

 gradually becomes shorter as the temperature rises. 



The exposure to light apparently produced abnormal rapidity 

 in the development of the eggs. In lot 18 this influence was scarcely 

 felt, while in lots 1, 4, and 6 it was very cogent, and it is evident that 

 exposure to light is chiefly influential under low temperatures. The 

 comparatively slow development of lots 25 to 27 apparently can be 

 laid only to temperature fluctuations obtaining in the incubator. 

 This fluctuation in the incubator sometimes consisted in the mainte- 

 nance of a lower minimum for a longer period than that which 

 obtained in the main part of the cellar. Such temperatures possibly 

 would exert a retarding effect upon egg development that would 

 not appear in the averaged readings of the thermometer. 



Even among the lots kept in the cellar under similar conditions 

 there were apparent exceptions to the rule that "the higher the 

 temperature the shorter the period of incubation." One such instance 

 is that of lots 15 and 16, in which two large series were used, yet 

 under temperatures differing but 0.1° F. there was a difference in the 

 average incubation periods of one and a half days. 



Among the individuals enumerated in Table VIII the maximum 

 egg stage was 27 and the minimum 5 days. The respective average 

 temperatures influencing the two individuals were 55° F. and 68° F., 

 and both were incubated in the cellar. It was possible only to estimate 

 an annual average incubation stage, and this was about 11 days. 

 It should be added that eggs have been observed in December to 

 incubate in a period exceeding 30 days, but it is unusual to find eggs 

 at this time of year. 



Experiments conducted in the cellar demonstrated that eggs in- 

 cubated as rapidly in arid as in humid surroundings, but submergence 

 in water lengthened the incubation period, even under equal tem- 

 peratures. 



Incubation on living roots. — During the years 1913, 1914, and 

 1915 biologic records were made on the living roots of young vines 

 of viniferse and vinifera X American hybrids. A series of gen- 

 erations were conducted during these three years, and incubation 

 was observed for each generation. In most cases immediately after 

 deposition the eggs were removed to an unifested root, but in some 

 they were allowed to incubate where they had been deposited. The 

 cages containing the experimental vines were all placed together in 

 one trench, and the temperature was alike in all of them. Table 

 IX indicates the incubation periods with reference to temperatures 

 and time of year. The years are not given, as in some instances 



