But as there are many species of oviparous animals, so must there also be several species of eggs; for every primordium is not alike fit to receive or assume every variety of animal form indifferently. Though we admit, therefore, that eggs in a general sense do not differ, yet when we find that one is perfect, another imperfect, it is obvious that they differ essentially from one another. Perfect eggs are such as are completed in the uterus, where they obtain their due dimensions before being extruded; of this kind are the eggs of birds. Imperfect eggs, again, are such as are prematurely excluded before they are of the full size, but increase after they are laid; of this description are the eggs of fishes, crustacea and mollusca; the primordia of insects, which Aristotle entitles worms, are farther to be referred to this class, as well as the primordia of those animals that arise spontaneously.
Moreover, although perfect eggs are of two colours, in other words, are composed of albumen and vitellus, some are still only of one hue, and consist of albumen alone. In like manner, of imperfect eggs, some from which a perfect animal proceeds are properly so called; such are the eggs of fishes; others are improperly so styled, they engendering an imperfect animal, namely, a worm, grub, or caterpillar, a kind of mean between a perfect and an imperfect egg, which, in respect of the egg or the primordium itself, is an animal endowed with sense and motion, and nourishing itself; but in respect of a fly, moth or butterfly, whose primordium it is potentially, it is as a creeping egg, and to be reputed as adequate to its own growth; of this description is the caterpillar, which having at length completed its growth is changed into a chrysalis or perfect egg, and ceasing from motion, it is like an egg, an animal potentially.
In the same way, although there are some eggs from the whole of which a perfect animal is produced by metamorphosis, without being nourished by any remains of the substance of the egg, but forthwith finds food for itself abroad, there are others from one part of which the embryo is produced, and from the remainder of which it is nourished:—although, I repeat, there are such differences among eggs, still, if we be permitted to conclude on the grounds of sense and analogy, there is no good reason wherefore those that Aristotle calls worms should not be spoken of as eggs; inasmuch as all vegetal principles are not indeed animals actually, but are so potentially, are true animal seeds, analogous to the seeds of vegetables, as we have already demonstrated in the particular instance of the hen’s egg. All animals are, therefore, either viviparous or oviparous, inasmuch as they all either produce a living animal in fact, or an egg, rudiment, or primordium, which is an animal potentially.
The generation of all oviparous animals may therefore be referred to that of the hen’s egg as a type, or at all events deduced from thence without difficulty, the same things and incidents that have been enumerated in connexion with the common fowl being also encountered in all other oviparous animals whatsoever. The various particulars in which they differ one from another, or in which they agree, either generally, or specifically, or analogically, will be subsequently treated of when we come to speak of the generation of insects and the animals that arise equivocally. For as every generation is a kind of way leading to the attainment of an animal form, as one race of animal is more or less like or unlike another, their constituent parts either agreeing or disagreeing, so does it happen in respect of their mode of generation. For perfect nature, always harmonious with herself in her works, has instituted similar parts for similar ends and actions: to arrive at the same results, to attain the same forms, she has followed the same path, and has established one and the same method in the business of generation universally.
Wherefore as we still find the same parts in the perfect or two-coloured egg of every bird, so do we also observe the same order and method pursued in the generation and development of their embryos as we have seen in the egg of the common fowl. And so also are the same things to be noted in the eggs of serpents and of reptiles, or oviparous quadrupeds, such as tortoises, frogs, and lizards, from all the perfect two-coloured eggs of which embryos are produced and perfected in the same manner. Nor is the case very different in regard to fishes. But of the manner in which spiders and the crustacea, such as shrimps and crabs, and the mollusca, such as the cuttlefish and calamary, arise from their eggs; of the conditions also upon which worms and grubs first proceed from the eggs of insects, which afterwards change into chrysalides or aurelias, as if they reverted anew to the state of eggs, from which at length emerge flies or butterflies—of the several respects in which these differ in their mode of generation from an egg, from what we have found in the hen’s egg, will be matter for remark in the proper place.
Although all eggs consisting of yelk and white are not produced and fecundated in the same manner, but some are made prolific through the intercourse of male and female, and others in some other way (as of fishes); and although there is some difference even in the mode in which eggs grow, some attaining maturity within the body of the parent, others continuing to be nourished and to grow when extruded, there is still no reason why an embryo should not be developed in the same precise manner in every egg—always understood as perfect—as it is in the egg of the hen. Wherefore the history which has been given of the evolution of the chick from the hen’s egg may be regarded as applicable to the generation of all other oviparous animals whatsoever, as well as to the inferences or conclusions which may be deduced from thence.
EXERCISE THE SIXTY-THIRD.
Of the generation of viviparous animals.
Thus far have we treated mainly of the generation of oviparous animals; we have still to speak particularly of the other species of generation, the viviparous, to wit, in which many things identical with those we have noticed in oviparous generation will come to be observed. These we have reduced into order, and here at length present for consideration. Even the parts that appear paradoxical and in contradiction with the current views of generation will, I believe, be found entirely in conformity with truth.
Among viviparous animals, man, the most perfect of all creatures, occupies the foremost place; after him come our ordinary domestic animals, of which some are soliped, such as the horse and ass; others bisulcate, as the ox, goat, sheep, deer, and hog; others digitate, such as the dog, cat, rabbit, mouse, and others of the same description; from the modes of whose generation a judgment may be formed of that of all other viviparous animals. Wherefore I shall propose a single genus, by way of general example or type, as we did in the case of the oviparous class; this made familiar to us, will serve as a light or standard, by means of which all the others may be judged of by analogy.