other special ends ; it therefore happens that for the disposition of material, and other requisites, some parts are variously en- gendered before others, some of them being begun earlier but completed at a later period, some being both begun and perfect- ed at an earlier period, and others being begun together but per- fected at different times subsequently. And then the same order is not observed in the generation of all animals, but this is va- riously altered ; and in some there is nothing like succession, but all the parts are begun and perfected simultaneously, by metamorphosis, to wit, as has been already stated. Hence it follows, in fine; that the primogenate part must be of such a nature as to contain both the beginning and the end, and be that for whose sake all the rest is made, namely, the living principle, or soul, and that which is the potential and genital cause of this, the heart, or in our view the blood, which we regard as the prime seat of the soul, as the source and perennial centre of life, as the generative heat, and indeed as the inherent heat ; in a word, the heart is the first efficient of the whole of the instru- mental parts that are produced for the ends of the soul, and used by it as instruments. The heart, according to Aristotle, I say, is that for which all the parts of animals are made, and it is at the same time that which is at once the origin and fashioner of them all.
EXERCISE THE FIFTY-SIXTH.
Of the order of the parts in generation as it appears from observation.
That we may now propose our own views of the order of the parts in generation as we have gathered it from our observa- tions, it appears that the whole business of generation in all animals may be divided into two periods, or connected with two structures : the ovum, i. e. the conception and seed, or that, whatever it be, which in spontaneous productions corre- sponds to the seed, whether with Fernelius it be called "the native celestial heat in the primogenial moisture," or with Aristotle, " the vital heat included in moisture." For the con- ception in viviparous animals, as we have said, is analogous to
