The reasons that led me to select the hen’s egg as the measure of eggs in general have been already given: eggs are of little price, and are everywhere to be obtained, conditions that permit repeated study, and enable us cheaply and readily to test the truth of statements made by others.
We have not the same facilities in studying the generation of viviparous animals: we have rarely, if ever, an opportunity of dissecting the human uterus; and then to enter on the subject experimentally in the horse, ox, sheep, goat, and other cattle, would be attended with immense labour and no small expense; dogs, cats, rabbits, and the like, however, will supply those with subjects who are desirous of putting to the test of experiment the matters that are to be delivered by us in this place.
Fabricius of Aquapendente, as if every conception of a viviparous animal were in a certain sense an egg, begins his treatise with the egg as the universal example of generation; and among other reasons for his conclusions assigns this in particular:[330] “Because the study of the egg has the most extensive application, the greater number of animals being engendered from eggs.” Now we, at the very outset of our observations, asserted that ALL animals were in some sort produced from eggs. For even on the same grounds, and in the same manner and order in which a chick is engendered and developed from an egg, is the embryo of viviparous animals engendered from a pre-existing conception. Generation in both is one and identical in kind: the origin of either is from an egg, or at least from something that by analogy is held to be so. An egg is, as already said, a conception exposed beyond the body of the parent, whence the embryo is produced; a conception is an egg remaining within the body of the parent until the fœtus has acquired the requisite perfection; in everything else they agree; they are both alike primordially vegetables, potentially they are animals. Wherefore, the same theorems and conclusions, though they may appear paradoxical, which we drew from the history of the egg, turn out to be equally true with regard to the generation of animals generally. For it is an admitted fact that all embryos, even those of man, are procreated from some conception or primordium. Let us, therefore, say that that which is called primordium among things arising spontaneously, and seed among plants, is an egg among oviparous animals, i. e. a certain corporeal substance, from which, through the motions and efficacy of an internal principle, a plant or an animal of one description or another is produced; but the prime conception in viviparous animals is of the same precise nature, a fact which we have found approved both by sense and reason.
What we have already affirmed of the egg, viz. that it was the sperma or seed of animals and analogous to the seeds of plants, we now affirm of the conception, which is indeed the seed of an animal, and therefore also properly called ovum or egg. Because “a true seed,” according to Aristotle,[331] “is that which derives its origin from the intercourse of male and female, and possesses the virtues of both; such as is the seed of all vegetables, and of some animals, in which the sexes are not distinct, and is, as that which is first mingled from male and female, a kind of promiscuous conception or animal; for it has those things already that are recognised of both;” i. e. matter adapted to nourish the fœtus, and a plastic or formative and effective virtue. And so in like manner is a conception the fruit of the intercourse of male and female, and the seed of the future embryo; it therefore does not differ from an egg.
“But that which proceeds from the generant is the cause which first obtains the principle of generation, (i. e. it is the efficient cause,) and ought to be called the geniture,”[332] not the seed, as is commonly done both by the vulgar and philosophers at the present time; because it has not that which is required of both the concurring agents, neither is it analogous to the seeds of plants. But whatever possesses this, and corresponds to the seeds of vegetables, that too is rightly entitled egg and conception.
Further, the definition of an egg, as given by Aristotle,[333] is perfectly applicable to a conception:—“An egg,” he says, “is that the principal part of which goes to constitute an animal, the remainder to nourish the animal so constituted.” Now the same thing is common to a conception, as shall be made to appear visibly from the dissection of viviparous animals.
Moreover, as the chick is excluded from the egg under the influence of warmth derived from the incubating hen or obtained in any other way, even so is the fœtus produced from the conception in the uterus under the genial warmth of the mother’s body. In few words, I say, that what oviparous animals supply by their breast and incubation, viviparous animals afford by their uterus and internal embrace. For the rest, in all that respects the development, the embryo is produced from the conception in the same manner and order as the chick from the egg, with this single difference, that whatever is required for the formation and growth of the chick is present in the egg, whilst the conception, after the formation of the embryo, derives from the uterus of the mother whatever more is requisite to its increase, by which it continues to grow in common with the fœtus. The egg, on the contrary, becomes more and more empty as the chick increases; the nutriment that was laid up in it is diminished; nor does the chick receive aught in the shape of new aliment from the mother; whilst the fœtus of viviparous animals has a continued supply, and when born, moreover, continues to live upon its mother’s milk. The eggs of fishes, however, increase through nourishment obtained from without; and insects and crustaceous and molluscous animals have eggs that enlarge after their extrusion. Yet are not these called eggs the less on this account, nor, indeed, are they therefore any the less eggs. In like manner the conception is appropriately designated by the name of ovum or egg, although it requires and procures from without the variety of aliment that is needful to its growth.
Fabricius gives this reason for some animals being oviparous, for all not producing living offspring: “It is,” he says, “that eggs detained in the uterus till they had produced their chicks would interfere with the flight of birds, and weigh them down by their weight. Serpents would also be hindered in their alternate zig-zag movements by a multitude of eggs in the abdomen. In the body of tortoises, with their hard and girding shell, there is no room for any store or increase of eggs; nor would the abdomen of fishes suffice for the multitude of eggs they must spawn were these to grow to any size. It was, therefore, matter of necessity that those creatures should lay their eggs imperfect. It seems most natural that an animal should retain and cherish its conception in its interior until the fœtus it produces has come to maturity; but nature sees herself compelled, as it were, occasionally to permit the premature birth of various eggs, and to provide them, without the body of the parent, with the nourishment they require for their complete development. As to everything that refers to the evolution of the fœtus, all animals are engendered from an oviform primordium; I say oviform, not as meaning that it has the precise configuration of an egg, but the nature and constitution of one; this being common in generation, that the vegetal primordium whence the fœtus is produced, including the nature of an egg, corresponding in its proportions to the seed of a plant, pre-exists. In all vegetal primordia, consequently, whether eggs, or having the form of eggs, there are inherent the nature and conditions of an egg, properties which the seeds of plants have in common with the eggs of animals. The primordium of any animal, whatsoever, is therefore called seed and fruit; and in like manner the seed of every plant is spoken of as a kind of conception or egg.”
And this is the reason why Aristotle says:[334] “Animals that engender internally have something formed in the fashion of an egg after their first conception: there is a fluid contained within a delicate membrane, like an egg without the shell. And this is the cause why the disorders of the conception, which are apt to occur in the early period, are called discharges.” Such a discharge is particularly observed among women when they miscarry in the course of the first or second month. I have repeatedly seen such ova aborted at this time; and such was the one which Hippocrates has described as having been thrown off by the female pipe-player in consequence of a fall.
In the uterus of all animals there is consequently present a prime conception or primordium, which, on Aristotle’s testimony,[335] “is like an egg surrounded with a membrane from which the shell had been removed.” This fact will appear still more plainly from what is about to be said. Meantime let us conclude with the philosopher, “that all living creatures, whether they swim, or walk, or fly, and whether they come into the world with the form of an animal or of an egg, are engendered in the same manner.”