In spiritu praedicto, which I render 'in the aforesaid subject': because the spiritus, pneuma, or 'gas' that made according to Aristotle to en tô spermati aphrôdes te kai leukon, has been declared by St Thomas to be the 'proper subject' in which the 'formative power' inheres. Spiritus was a vague word to a mediaeval writer: it was fraught with suggestions high and divine. St Thomas would have shrunk from reducing spiritus, to the mysterious vehicle of the vis formativa seminis, to the banality of gas. But the pneuma of De gen. animal., II, ii, the authority on which he relied, is gas pure and simple.
As a piece of morphology, all this speculation about pneuma, aphros, spiritus, spuma, gas and foam, must be swept away. It is false, as we have seen the analogy of a bisected Annelid to be false. The cutting of a worm in two is no example of the generative process; and there is no such thing in any semen as this genetic gas. Chemical and microscopic examination of the mammalian semen reveal quite another structure and composition.
So far as biology sees it, what actually happens in conception is this: -- "Wherever they meet the female ovum, the male spermatozoa surround it, often in dense masses. Only one spermatozoon however effects an entrance into the ovum, after the following fashion. The tail is left behind, and the nucleated head with the centrosome passes into the ovum, generally as a place called the 'micropyle.' Certain changes have been going on in the ovum to anticipate this event, and the renewed nucleus of the ovum is awaiting developments. This is known as the 'female pronucleus.' Certain changes prepare the nucleated head of the spermatozoon for action, and what is known as the 'male pronucleus' results. The male pronucleus proceeds to fuse with the female pronucleus, and a new nucleus, the result of the combination, the 'segmentation nucleus' results. Thus the male element and the female element seem to take an equal part in the formation of the embryo: for immediately after the combined nucleus is formed, the work of segmentation and formation of the tissues goes on. Though fertilisation is effected by quite a microscopic quantity, one single spermatozoon entering the ovum, we must observe that an equally microscopic part of the ovum is fertilised: for the great bulk of what we call the ovum is made up of nutritive material, food-yolk, etc."
So far, so clear, much in advance of St Thomas. But concerning any vis formativa, directrix of this wonderful process of conception and development; and about the origin and function of soul, vegetative, sentient, and intelligent; we remain shrouded in the darkness of the thirteenth century. We want a new treatise De anima, to be written by some Aquinas modernus, who shall be at once a profound Aristotelian and an expert biologist, and shall consecrate his life to this one study of soul. He should not neglect the mistaken biology of the original Aquinas and Aristotle. The mistakes of great minds are suggestive: they are far-reaching in the history of thought. Thus, as one reads Aristotle, De gen. animal., II, ii, the memory is carried to St John's Gospel, iii, 5; vi, 63; and his first Epistle, v, 8: ean mê tis gennêthê ex hudatos kai pneumatos -- to pneuma esti to zôopoioun -- to pneuma kai to hudôr kai to haima.
Of God and His Creatures: 2.88