External Factors that Lnfiuence Growth 261 
mal growth takes place at 25° C. At this temperature tadpoles 
develop as far in 24 hours as do those at 16° in two days. 
I have found that the young tadpoles of Rana palustris kept at 
a temperature of 2 to 2.5 C. develop scarcely at all, and may be 
kept for at least a month in practically the same condition. 
Rauber states that the eggs of minnows and salmon, which 
develop during the winter season, will not grow above 12-15%, 
but do grow at o°. On the other hand, it is known that those 
fish that deposit their eggs in summer develop faster at tempera- 
tures higher than these. 
The optimum temperature for development of the hen’s egg 
is about 38° C., but it will slowly develop at 25° C. The maxi- 
mum point at which development takes place normally is 42°. 
The following table shows the rate for the chick : —- 
Temperature . . 347% 35° 36° 37° 38° 39° 40° 41° 
Index of development 0.65 0.80 0.72 ? (1.00) 1.06 1.25 1.51 
Most other animals have a much greater range at which normal 
development occurs. It is evident that for each species of ani- 
mal there is an optimum temperature for growth, which is very 
different in different species. Somewhat above the optimum 
temperature, growth may be more rapid although less normal; 
but as higher temperatures are reached, the development is inter- 
fered with very greatly. A much wider range of possibilities exists 
for lower temperatures. This difference is probably due to the 
coagulation of the protoplasm. Slightly above the optimum 
temperature the colloidal substances of the protoplasm become 
coagulated. It requires much greater change in the other direc- 
tion, z.e. for lower temperatures to coagulate the colloids. In 
many cases a freezing temperature is necessary to produce this 
effect, but even this temperature is not injurious to many animals. 
Effects of Light on Growth 
In studying the effects of light on growth one must be careful 
to exclude the heat rays. This can be done by interposing a 
screen containing solutions that absorb the heat rays and allow 
