Heat.—­To keep animal life, once started, alive and growing, we need:  First, a suitable surrounding temperature.  Second, a fairly constant proportion of water in the body substance.  Third, oxygen.  Fourth, food.

Now, a fertile egg is a living young animal and as such its wants should be considered.  We may at once dispose of the food problem of the unhatched chick, by saying that the food is the contents of the egg at the time of laying, and as far as incubation is concerned, is beyond our control.

In consideration of external temperature in its relation to life, we should note:  (A) the optimum temperature; (B) the range of temperature consistent with general good health; (C) the range at which death occurs.  Just to show the principle at stake, and without looking up authorities, I will state these temperatures for a number of animals.  Of course you can dispute the accuracy of these figures, but they will serve to illustrate our purpose: 

External  External   External  Internal  Internal
Optimum   Healthful   Fatal    Optimum    Fatal
Point     Range      Range     Point     Range

Man 70 0 to 100 50 to 140 98 90 to 106

Dog 60 70 to 140 70 to 140 101 95 to 110

Monkey 90 30 to 140 30 to 140 101 95 to 108

Horse 80 20 to 120 20 to 120 99 95 to 105

Fowl 80 20 to 140 20 to 140 107 100 to 115

    Newly hatched
    chick 90 70 to 100 40 to 120 108 100 to 115

    Fertile egg
    at start of
    incubation 103 32 to 110 31 to 125 103 31 to 125

    Egg incubated
    three days 103 98 to 105 80 to 118 103 95 to 118

    Egg incubated
    eighteen days 103 75 to 105 50 to 118 106 98 to 116

This table shows, among other things, that we are considering in the chick not a new proposition to which the laws of general animal life do not apply, but merely a young animal during the process of growth to a point where its internal mechanism for heat control, has power to maintain the body temperature through a greater range of external temperature change.

In the cooling process that occurs after laying the living cells of the egg become dormant, and like a hibernating animal, the actual internal temperature can be subjected to a much greater range than when the animal is active.  After incubation begins and cell activity returns, and especially after blood forms and circulation commences, the temperature of the chick becomes subject to about the same internal range as with other warm blooded animals.