Reference for Bekhorot 14:31

אמר רב ספרא

Said R'Safra: It was really the seed of a dee which sought to couple with a hind, but since the latter's womb is narrow and it is unable to copulate, the deer, therefore, seeks to couple with a fallow-deer, releasing its semen into the latter's womb.<span class="x" onmousemove="('comment',' The semen, however, owing to the delay in copulation, has meanwhile hardened, and although it enters the womb, owing to its congealed state, it has no effect and issues later in the animal's excrements, in the form of ball concretions.');">¹⁵</span>

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