During incubation, the embryo converts the content of the egg into its body. This process requires energy, that the embryo gets by burning the fat in the yolk. This burning of fat (and to a much less extent carbo hydrates) produces the energy that the embryo requires, and next to that carbon dioxide, metabolic water and metabolic heat

In an optimum incubation process, the embryo converts as much energy as possible into its body. As a result, the majority of the yolk will be used by the time of hatching, resulting in a small residual yolk. Next to that, the embryo will produce a high amount of metabolic heat, as a waste product of its metabolism.

The rate of conversion of energy into embryo seems to be dependent on a combination of temperature and oxygen. There is still a lot unknown about what drives this development, and what limits it. However, if we want to have as much development as possible in the embryo, we should have as much embryo development and as little residual yolk sac as possible left over at the moment of hatching.

We can measure this development by substracting the residual yolk from the body fo the day old chick. By measuring the residual yolk and the yolk free body mass we can have a good indicator for its development.

Another, more quick and more animal friendly way is to stretch the day old chick over a ruler and measure its length from tip of the beak to tip of the middle toe. This measurement has shown to be very constant and repeatable.

Research has shown a consistent and positive correlation between chick length as day-old and growth performance later in life, indicating that development can be measured in this way and that it has a predictable value for later performances.