Halakhah for Niddah 86:46
ושרץ לא מטמא במשהו
and to semen in general.<span class="x" onmousemove="('comment',' Sc. it referred to the form of uncleanness appropriate to each. A dead creeping thing can never convey uncleanness unless its bulk is of the prescribed size, while semen, when it concerns the man who had emitted it, may convey uncleanness, however small its quantity. ');"><sup>34</sup></span>
Sefer HaChinukh
From the laws of the commandment is that which they, may their memory be blessed, said (Niddah 43b) that the measure for one who touches [it] is like [the size of] a lentil, and for one who [experiences] the emission is with the smallest amount. And it is one, whether he [experiences] the emission accidentally or volitionally, he is impure. And the seed of a young child does not render impure. And red seed does not render impure, but rather [only] white. And any seed that a man does not feel - not at the beginning and not at the end - is not impure. And one who has [improper] thoughts at night, and saw in a dream that he had sexual relations and got up and found his flesh [to be] warm, is impure - even though he did not [knowingly experience] semen. And the rest of its details are elucidated in Tractate Zavim (see Mishneh Torah, Laws of Offerings for Those with Incomplete Atonement 1).
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