Chullin 126 - An Egg of a Reptile
The Torah lists eight special reptiles , such as lizard and snail, and it also includes other small animals in the list, such as weasel and mouse, calling them the collective name of "crawlers," but we will refer to them for simplicity as reptiles.
As a rule, a "protector" of a substance conveys impurity, but only when the substance itself is accessible to touch. For example, an egg of a reptile in which an embryo has been formed is pure and does not convey impurity, because the embryo is not accessible to touch. On the other hand, if the eggshell is punctured even a small amount, it does convey impurity.
There was a species of a mouse that does not reproduce sexually but is generated spontaneously from the earth, much as maggots from decaying food. If this mouse died while it was halfway formed, with one side of it flesh and the other still earth, one who touches the flesh of it is impure, but one who touches the earth of it is pure. Rabbi Yehudah says, "One who touches the earth opposite the flesh is also impure."
Art: Willem Van Aelst - Still-Life with Mouse and Candle