OUTLINE OF DEVELOPMENT, CHRONOLOGY 65 



varies with the freshness of the egg; it is relatively short in eggs 

 that are newly laid, and long in eggs that have remained qui- 

 escent some time after laying. It is obvious that the latent 

 period will form a more considerable portion of the entire time 

 of incubation in early than in late stages. Hence the difficulty 

 of classifying embryos, particularly in the first four or five 

 days of incubation, by period of incubation. Eggs procured from 

 dealers usually show such great variations in degree of develop- 

 ment, at the same time of incubation, that it is quite impossible 

 to grade them with any high degree of accuracy by time of incu- 

 bation. It is stated also that the rate of development varies 

 considerably at different seasons, other factors being constant. 

 But this has not been found to be a serious matter in my own 

 experience. 



Variations in temperature, either above or below the normal, 

 also seriously affect the rate of development, and produce abnor- 

 malities when extreme. If the temperature be too low, the rate 

 is slower than normal; if too high, the rate increases up to a 

 certain point, beyond which the egg is killed. 



The physiological zero, that is the temperature below which 

 the blastoderm undergoes no development whatever, has been 

 estimated differently by different authors. Some place it at 

 about 28° C, others at about 25°; Edwards places it as low as 

 20-21° C. At the last temperature, apparently, a small percent- 

 age of eggs will develop in the course of several days to an early 

 stage of the primitive streak, but most eggs show no perceptible 

 development. In very warm weather, therefore, the atmos- 

 pheric temperature may be sufficient to start eggs. The follow- 

 ing table is given by Davenport based on Fere's work: 



The index of development represents the proportion that the 

 average development at a given temperature in a given time 

 bears to the normal development {i.e., development at the normal 

 temperature for the same time). There is an increase in the rate 

 up to 41°; a maximum temperature, which cannot be much 

 above 41°, causes the condition of heat-rigor and death. 



There would seem to be no better way to determine the normal 

 temperature for incubation than by measuring the temperature 



