Reference for Chullin 148:5

לא הילכו בו אלא על עסקי שחיטה בלבד

and the [sciatic] nerve.<span class="x" onmousemove="('comment',' I.e., the fat and the sciatic nerve of the foetus are forbidden as in an ordinary animal, and there is no dispute about these (Rashi) . According to R. Gershom, all agree that the fat and the nerve of the foetus are permitted.');">²</span>

Jastrow