III. 5 



HEAT 



103 



PERCENTAGE 

 OF NORMAL 

 DEVELOPMENT 



60 



50 



40 



30 



20 



24 



26 



30 



FIG. 54. The index of development (percentage of normal develop- 

 ment) for the egg of the Hen at temperatures varying from 20 C to 

 30-75 C. (After Edwards, 1902.) 



4.90 



4.4020 21 22 23 24 25 26 21 



RELATION OF TEMPERATURE TO GROWTH OF BLASTODERM 



PIG. 55. Growth of the blastoderm of the Hen's egg independently of 

 the appearance of the primitive streak, at low temperatures. (After 

 Edwards, 1902.) 



