Reference for Chullin 154:22

אבל יש עמה ולד בין קשורה בולד בין אין קשורה בולד אין חוששין לולד אחר

Why? - R'Ika the son of R'Ammi said: Because the majority of animals give birth to something which is holy as a firstling whereas a minority of animals give birth to something which is not holy as a firstling, to wit, a nidmeh.<span class="x" onmousemove="('comment',' Heb. 'like, similar to'; e.g. a ewe which gave birth to what looked like a kid, or a goat which gave birth to what looked like a lamb. This is not holy as a firstling, v. Bik. II, 5.');">¹²</span>

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