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 Féré’s work: 
Temperature 34° 35° = 386° 87° 338° 39° = 40° 41° 
Index of Development 0.65 0.80 0.72 1:00) 1-06 1225, Arb 
The index of development represents the proportion that the 
average development at a given temperature in a given time 
bears to the normal development (7.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 
