Chick development question

[FrenchToast]

At what stage does the chick uptake the yolk of the egg? Is this during the last few days of incubation?
I"m not talking about them obsorbing the yolk after hatch.
The reason I'm asking is I've had chicks that hatched or should I say made it to lockdown then never hatched and when I opened them up there was the yellow yolk hanging out of their bottom. What went wrong or when did they stop developing?
Thanks

 

[Tala]

this might help: http://edis.ifas.ufl.edu/aa204

 

[pigfarmer]

Great link................... I bookmarked it
Thanx

 

[trilyn]

Excellent link, I bookmarked it also-thanks!