66 THE DEVELOPMENT OF THE CHICK 



of eggs incubated by the hen throughout the entire period of 

 incubation. This has been done very carefully by Eyclesh3mier, 

 who finds the internal temperature of such eggs to be as follows: 



Temperature of egg 103.0 102.4 103.0 103.0 103.0 



The temperature of the hen is seen to be somewhat higher 

 than that of the eggs. In an artificial incubator where 85 % of 

 the fertile eggs hatched on the twentieth and twenty-first da3^s, 

 the temperatures were as follows: 



Day of incubation 

 Temperature of incubator 

 Temperature of egg 



Day of incubation 

 Temperature of incubator 

 Temperature of egg 



Day of incubation 

 Temperature of incubator 

 Temperature of egg 



Day of incubation 

 Temperature of incubator 

 Temperature of egg 



It would be possible then to establish a normal rate of develop- 

 ment, by using perfectly fresh eggs incubated at a normal tem- 

 perature. In practice I have found that the times given in Duval's 

 atlas are approximately normal, and these are, therefore, adopted 

 so far as given. But even under the best conditions the varia- 

 tions are sufficient to prevent close grading of stages by time of 

 incubation in the first three days. This may be due to differences 

 in the grade of development at the time of laying, owing to varia- 



