What explanation can be given, after all these facts, of the origin of double monsters ? On this subject, three hypotheses ought to be mentioned : 1. The double monster has been supposed to have proceeded from two distinct embryos, which have become united in the course of development ; 2. It has been held to have originated in a single germ, which has become double, or has been subdivided ; and 3. The germ has been regarded as abnormally compound from the first, implying that the organs and parts composing the double mon ster are at once produced from this germ, without either separation or coalition of irs parts other than belong to the natural process of developement. On the comparative merits of the first and second hypotheses, as parts of the general doctrine of monsters, one of the most interesting physiological discussions extant is recorded in the Menzoires de 1 'Aca denzie des Sciences de Paris, between 1724 and 1743. The chief disputants were Lemery and Winslow ; the contest lasted nineteen years ; it engaged the attention of all anato mists, and called forth writings by Haller and a crowd of authors of less note, and was only terminated by the death of Lemery. Every argument that could be founded on the know ledge of those days was brought forward, and the subject was, for the time, utterly exhausted ; but the facts accumulated in later years have furnished such volumes of additional evidence, that the same question, between original and acquired monstrosity, as far as it relates to double monsters, may even now claim to be discussed.
It is certain that two ova may be formed in one Graafian vesicle (Von Baer, Bischoff, Bidder). The equally well-known fact, that the common fowl sometimes produces double yolked eggs, naturally led at one time to the opinion that the formation of double monsters might be attributed to the developement and subsequent union of the embryo in each yolk ; an opinion which has been adopted by some on very insufficient grounds, as it does not appear to be warranted by any direct observ ations made upon the result of the incubation of double-yolked eggs, and is at variance with much of what is known of the structure and mode of union of the two embryos composing a double monster.* C. F. Wolff has distinctly affirmed, that a double-yolked egg is equivalent to a double ovum ; that the produce of its incubation would be twins ; and that a double monster can only proceed from a single yolk contain ing a double germ. Examples of double em bryos of birds sometimes occur at the full period of incubation, in which both are com plete, and there is no union, excepting at the umbilicus. It is barely possible that each of these embryos may have been developed from a separate yolk, and that in the course of incubation the two yolks have come to coa lesce, in consequence of external pressure, or other causes. Towards the conclusion of the period of incubation, when the yolk usually enters the abdomen of the foetus, we may suppose, in the case before us, a partial en trance of the common yolk into the abdomen of each embryo, and thus, upon the subse quent contraction of the umbilical aperture, the union of the two embryos may be effected.
We learn, from the accurately-detailed ob servation of C. F. Wolff, previously referred to in my monograph, that two completely separate foetuses may be formed in the bird's egg upon a single yolk, and within a single germinal and vascular area. The egg, in this instance, had been incubated six days, and both the en r bryos were at once so com plete and so distinct, that there is no reason to believe they would have been united till the period when the entrance of the yolk into the abdomen of both, and the contraction of the umbilical apertures, had brought them together.
In a dozen double-yolked eggs, which Prof. Allen Thompson brought to incubation, he never succeeded in obtaining a double mon ster, nor even two embryos, at the full period, from any of them. In several instances he found that one yolk only had been productive.
All this proves that such double-yolked eggs may produce twins, but that the forma tion of a double monster is not dependent on them. It is highly probable that in the same manner, in Mammalia, the arrival in the uterus of two impregnated ova, in close proximity with one another, will be attended with the production, not of a double monster, but of twins.
The complete fusion of these twins seems to me quite impossible. One of my chief arguments against the hypothesis of fusion of originally separate germs is the important fact, which I derive from my own investi gations and from those of others, that double monsters form one series, among whose seve ral members the degrees and modes of devia tion from singleness gradually increase, and pass, without one abrupt step, from the ad dition of a single ill-developed limb to the nearly complete formation of two perfect beings. Now if this be true, no hypothesis can he acceptable if it do not plausibly ex plain the origin of the whole series of double monsters, or if, though it may suffice to ex plain the facts in one part of the series, those in another part are opposed to it. And here is a fair objection against the hypothesis of fusion of two originally perfect and separate embryos. Grant that we might explain by it the formation of several of the more perfect instances of duplicity; still, if the same hypo thesis is altogether opposed by the simpler forms of duplicity, it is surely not tenable. For example, it cannot account for the exist ence on a child's sacrum of a shapeless mass, containing an isolated portion of intestine, as in Mr. Hanley's case. And still less can it explain the existence of a superfluous limb ; fur the limbs are mere off-shoots, and are produced at so late a period, that if we could imagine two embryos to come in contact by their shoulders or pelvis, and a fusion of those parts to take place, we should still have to explain how one of them, leaving only an arm or a leg behind him, could for the rest of his substance, head, trunk, and all, wholly dis appear.