Reference for Niddah 61:31

אמר רב פפא

are] tied up and whose orifice is turned upwards and yet it is doubtful whether [the things] would be preserved or not, whereas the Holy One, blessed be He, fashions the embryo in a woman's internal organ that is open and whose orifice is turned downwards and yet it is preserved. Another exposition: If a man puts his things on the scale of a balance, the heavier they are the lower the scale descends, whereas the Holy One, blessed be He, [fashioned the woman in such a manner that] the heavier the embryo the higher it rises.<span class="x" onmousemove="('comment',' Beginning in the lowest chamber at conception it rises steadily to the highest, as stated supra. ');">²⁶</span>

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