I thought the shell was necessary to supply calcium for bone development and as the calcium gets used, the shell becomes more porous and therefore easier to pip through.
Eta: "During development, the chick embryo mobilizes the calcium it needs from two extraembryonic sources, first the yolk and then the eggshell." Source
Eta: "During development, the chick embryo mobilizes the calcium it needs from two extraembryonic sources, first the yolk and then the eggshell." Source
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