This crude morphology takes up a chapter in Aristotle, De gen. animal., II, ii, e.g., esti men oun to sperma koinon pneumatos kai hudatos, to de pneuma estin thermos aêr . . . . dio hugron tên phusin hoti ex hudatos, pachu de kai leukon dia to memichthai pneuma . . . . aition de tês leukotêtos tou spermatos hoti estin hê gonê aphros, ho de aphros leukon. In the following chapter (chap. iii) we read that the heat of this 'gas' (pneuma), or 'hot air' (thermos aêr), contained in the frothy mass of the semen (emperilambanomenon en tô aphrôdei) is the generative element, -- poiei gonima ta spermata. The heat "is not fire, nor any such elemental power, but is analogous to the element of which the stars are made." It is otherwise described as "the quality of the gas," hê en tô pneumati phusis: it is once more the vis formativa, or virtus seminis of St Thomas. Whether this seminal power, supplied by the male, is (or becomes) the sentient soul, according to the opinion just refuted, or rather leads to the sentient soul being produced, as St Thomas proceeds to argue, makes the question discussed in the text. Aristotle comes not far short of saying that it is the sentient soul.

Of God and His Creatures: 2.88