Theses medicae de divina et admiranda hominis in utero formatione

...blood, as if attracted from the whole body through the seminal vessels of the testicles, and converted into a white fluid, and suffused with much spirit: whence from the immoderate excretion of it, the dissolution, cooling, and weakness of the body follows.

XV.

Others have thought it to be a fluid from the whole body and its parts: whose opinion Aristotle learnedly refutes, and we reject it with him.

XVI.

The matter of the seed, whether it be the residue of the ultimate nourishment, or a portion of fatter and purer blood, is drawn from the whole body, but especially from the principal parts, namely the liver, heart, and brain, which elaborate their nourishment more perfectly than the others, into the testicles through the seminal vessels.

XVII.

In the testicles themselves, however, the seed is truly effected and attains its ultimate perfection, partly by the heat of the surrounding parts, partly also by the innate power and faculty of the testicles themselves: for when dissected, they are found full of seed, which they had prepared for their own nutrition; but it is likely that it acquires a certain preparation in the spermatic vessels themselves.

XVIII.

We cannot approve of Aristotle’s opinion on the use of the testicles, by which he contended that they are like a hanging weight, and that he thought the spermatic vessels, to which he seems to have attributed all the power of generating seed, are the stabilizing [force], and therefore they were made not for necessity, but only for the sake of a better use.

XIX.

For we count them entirely among the principal parts of the body, insofar as they serve for the preservation of the species and provide some use common to the whole. For when they are removed, the strength, temperament, and own substance of the whole body is overturned and changed. As Eunuchs declare this.

XX.

The cause of this change seems to be the diminution of heat alone, in which they abound, and which they communicate to other particles.

XXI.

The male seed, having been sent into a well-tempered womb, effected with decent conformation, and well disposed, is mixed with the female [seed], and from the mixture of both, something one is constituted, which to Aristotle...