The vital principle in a man, or in anything else, is at the end, when it normally issues, of the same nature as it was in the beginning. If it is at perfection a substantial primary form, it always was such--a substantial form cannot issue from an accidental form. If the substantial form is the form of the cells in the completed organism, it was such before that organism was perfected, unless it replaced a lower substantial form; but there is, we repeat, absolutely no need for such a secondary form at the beginning. If the cells of the embryo (not the infused germ-cells, which are not the embryo) had a _forma corporeitatis_, or _cellularis_, or whatever you wish to call it, the human soul when it did come would not confer primal existence, would not be a _forma substantialis_, but an accidental form. "In proof of which," says St. Thomas,[55] "we must consider that a substantial form differs from an accidental form in this, that an accidental form does not give being simply, but such or such being; as heat does not give being simply, but heated being. So when an accidental form comes in, a thing is not said to come into existence or to be generated, simply, but to become such or such an object, or to find itself in such or such a condition. So, also, when an accidental form disappears, a thing is not said to be destroyed simply, but only to a certain degree. A substantial form, however, gives being simply; and therefore by its advent a thing is said to be generated simply, and by its recession to be destroyed simply. If, therefore, it happened that any substantial form other than the intellectual soul preëxisted in matter, by which the subject of that soul would come into actual being, it would follow that the soul would not confer being simply, and therefore would not be a substantial form; also that the coming of the soul would not be a generation simply, but only _secundum quid_--all of which is evidently false." Again, St. Thomas says:[56] "Some tell us the vital acts that appear in the embryo are not from the soul, but from the soul of the mother, or from the primitive force in the semen. Both these statements are false." [55] I, q. 76, corp. [56] Ia, q. 118, a. 2, ad 2. An application of the opinion offered here--that is, that the human soul is infused at the instant of conception--to multiple and monstrous embryos offers no real difficulty. There are two kinds of human twins--those from two distinct ova and those from one ovum. Two ova may come from one or different ovaries, or even from one Graafian follicle, be fertilized at the same time and develop synchronously. If the ova are placed at some distance apart in the uterus, two placentas appear; if the ova are near each other the placentas may fuse, but their circulations do not. Each child will have its own fetal envelope.