The navel of the chick closes in the last hours of the incubation process, when the process of uptaking the yolk is completed. A well closed navel is important as it the navel opening is a pathway for bacteria to enter the body cavity. Poorly closed navels will result in a higher percentage of mortality due to navel/yolk sac infections.
String navels are characterised by a piece of membrane that is dried up and emerges from the navel as a small string. Removing the string manually will worsen the condition, as the navel will be opened and the bacteria have a free enterance.
String navels are typically occurring when the hatching process goes to slow, usually as a result of a too low temperature. The embryo is not very actively pulling in the residual yolk sac due to the low temperature, and the navel will be closed with the last part of the membrane still being outside of the body cavity.
The low temperatures are not necessarily characterised by low machine temperatures. Excessive spraying or too much ventilation at the end of the hatching process can result in local coolling during the final stages of the hatching proces, which will result in an increase in string navels without an actual low temperature recorded by the machine. Also low fertility rates or not fully loaded hatchers will induce the incidence of string navels, as the embryonic heat production in the machine will be less.