INCUBATION 
ture be lowered, more food substance is combined with oxygen 
to keep up the warmth of the body, while, if the external 
temperature be raised, the body temperature is kept low by 
the cooling effects of evaporation. This occurs in mammals 
chiefly by sweating. Birds do not sweat, but the same effect 
is brought about by increased breathing. Now, the chick 
gradually develops the heat producing function during incuba- 
tion, until towards the close of the period it can take care of 
itself fairly well in case of lowered external temperature. The 
power to cool the body by breathing is not, however, granted 
to the unhatched chick, and for this reason the incubating egg 
cannot stand excess of heat as well as lack of it. 
The practical points to be remembered from the above are: 
First: Before incubation begins, eggs may be subjected to 
any temperature that will not physically or chemically injure 
the substance. 
Second: During the first few days of the hatch, eggs have no 
appreciable power of heat formation and the external tempera- 
ture for any considerable period of time can safely vary only 
within the range of temperature at which the physiological 
process may be carried on. 
Third: As the chick develops it needs less careful guarding 
ceeineh cooling, and must still be guarded against over-heat- 
ng. 
Fourth: It should be remembered, however, that eggs are 
very poor conductors of heat, and if the temperature change 
is not great several hours of exposure are required to bring 
the egg to the new temperature. , 
Temperature is the most readily observed feature about 
natural incubation and its control was consequently the first 
and chief effort of the early incubator inventors. 
A great deal of experimental work has been done to deter- 
mine the degree of temperature for eggs during incubation. 
The temperature of the hen’s blood is about 105 to 107 degrees 
F. The eggs are not warmed quite to this temperature, the 
amount by which they fail to reach the temperature of the 
hen’s body depending, of course, upon the surrounding tem- 
perature. 103 degrees F. is the temperature that has been 
generally agreed upon by incubator manufacturers. Some of 
these advise running 102 degrees the first week, 103 degrees 
the second, 104 degrees the third. As a matter of fact it is 
very difficult to determine the actual temperature of the egg 
in the box incubator. This is because the source of heat is 
above the eggs and the air temperature changes rapidly as 
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