Reference for Chullin 147:37

ולהאי תנא דפריך טרפה דשחיטתה מטהרתה מנא ליה

Now if it can be said [that the slaughtering of the foetu in its dam's womb renders it valid], then this also had a time when it was fit [for slaughtering], for a man might put his hand into the womb and slaughter it there [before it was rendered trefah]! - Raba said to him, Render: 'an animal that was formed<span class="x" onmousemove="('comment',' I.e., from the very beginning of the development of the embryo it was trefah, e.g. it was formed with five legs which renders it trefah, cf. supra 58b.');">¹⁸</span>

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