The development of an embryo is driven by the temperature of the egg. This so-called embryo temperature (the temperature inside the egg), is the result of the balance between heat production of the embryo and the heat loss of the egg to its environment.

The heat production of the embryo depends on:
- the line or breed used
- the size of the egg (or embryo)
- the moment of incubation
- the actual temperature of the egg

the heat loss depends (mainly) on:
- the temperature of the air
- the air velocity
- the evaporation of water from the egg shell

The temperature of the embryo can be determined by measuring the temperature of the egg shell with an infra-red ear thermometer. The goal for embryo temperature must be 100-100.5^oF for all eggs in the machine, at any given moment during the incubation process until 20 days. Due to the activity of the embryo during the actual process of hatching, the temperature of the embryo rises towards approximately 102,5^oF from day 20 onwards.