Halakhah do Kapłańska 3:4

And they, may their memory be blessed, also taught us (Chullin 75a) about the law of the embryo, that [the Torah] permits it with four benchmarks (its two and the two of its mother) - such that if its mother becomes 'torn,' one can slaughter the embryo and it is permissible; as it is not the 'thigh of its mother.' And that which they said (Chullin 75b) that if it hovered over the ground it requires slaughter; it is only because of appearances that we need to slaughter it. And therefore we do not disqualify it with pausing, pressing and the other types of [causing it to be] 'torn.' And there are three forbidden fats in a beast that have a liability of excision: that on the innards; that on the kidneys; and that on the flanks. And the understanding of the three of them is [found] with great elucidation in its place in the Gemara (Chullin 93a). And more generally, they, may their memory be blessed, said in Chullin 93a, "Chelev that meat surrounds is permissible; as [the Torah] stated, 'upon the flanks,' and not 'within the flanks.'"