If we do a break out on the hatch debris, we normally qualify the non-hatched chicks as dead in shell. However, we sometimes find a number of chicks that failed to hatch, They have completed the developmental process, have developed the pipping muscle, have (almost) completed the uptake of the residual yolk and the closure of the navel, are still alive but were not able to escape from the egg shell.
When we observe an increased number of these chicks during a break out, it might indicate that the hatch has been pulled too early, which didnt give the last chicks enough time to complete the hatching process.
It might also be that by itself the hatch was pulled on time, but the temperature in the last period of the hatching process was getting too low. This low temperature slows down the last chicks too much which prevents them from hatching and results in a dragging hatch. As the chick will also have a problem with completing the closure of the navel, more string navels might occur.
The cause might be a direct drop in machine temperature, but it might also be that the ventilation of the hatchers is increased too early, before the hatching process has been completed. This brings more cold and dry air into the machine, resulting in a cooling effect for the last eggs, especially if the sprayer comes in to compensate for the lack of moisture.
Nowadays we see often that the ventilation during hatching is controlled through the Carbon Dioxide level. When this is done to strictly, we might see an increase of the number of "alive in shell" as well. Carbon dioxide levels are increasing when the chicks are emerging from the shell, more or less similar with the increase in moisture production. Setting the machines on a fixed and low level of carbon dioxide during hatching will force the machine to open the ventilation earlier than normal to remove the carbon dioxide and with it also the humidity. By itself the low carbon dioxide levels will not have a negative effect, but the extra ventilation will bring in extra cold (and dry) air. Especially when the machine is set on an increasing relative humidity level to anticipate on the moisture that is coming from the hatching chicks, the machine will find the air to dry and will start spraying, making the last eggs hatching even colder.
When we run the hatchers on carbon dioxide control, we have to realise that asking the machine to ventilate more to control the carbon dioxide might result in a local cooling, making it difficult for the last chicks to hatch. The result will be a dragging hatch, more string navels and navel problems and more non-hatched eggs that can be categorized in a break out as: "alive in shell".