974 



TERATOLOGY. 



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 enrbryos 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 be 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. 



It was a main objection against the doctrine 

 of Lemery, that if two germs came in contact 

 by accident (as he supposed), they could not 

 exhibit any regularity in their mode of attach- 

 ment, but faces would be forced into chests, 

 abdomens into spines, and so on. The mo- 

 derns, who adopt the same hypothesis, sup- 

 pose that the ova come in contact not by ac- 

 cident, but by an attraction de soi pour sot, of 

 which the influence is, that the two ova being 

 by accident set face to face or back to back, 

 o" in any other way similia similibus, will be 

 drawn to each other, and will unite by similar 

 parts. But, with all respect for the authority 

 of M. Geoffroy St. Hilaire and his disciples 

 (as Mr. Paget elegantly said in his abstract 

 of my monograph upon double monsters), 

 who regard this as " la regie supreme de 

 tous les arrangements et de toutes les mo- 

 difications organiques chez les etres com- 

 poses," I confess that I can find no good 

 evidence that such an attraction exists. I 

 can see in it nothing more than a very happy 

 expression of a fact, which it in no wise ex- 

 plains. The extraordinary notion of MM. 

 Delpech and Coste, that such an attraction 

 may be the result of electric currents, is 

 certainly no evidence of its existence, this 

 being entirely imaginary. And the reasoning 

 in its favour seems no better than the facts, 

 for I can find nothing but this kind of circle: 

 monsters adhere by similar parts, therefore 

 there is an attraction de soi pour soi; there is 

 then such an attraction, and therefore double 

 monsters so adhere. I believe, therefore, that 

 such an attraction is hypothetical ; and if it 

 be so, surely the hypothesis which involves it 

 and an accident as essential elements is less 

 probable, as well as less sufficient, than that 

 which I maintain. It is scarcely better than 

 Lemery's, of mere accident ; for it requires 

 not only the accident of a particular position 

 of the ova, but that of their being of the same 

 sex, which has been said previously to be 

 the general rule for double monsters. After 

 all this, we conclude with the words of the 

 learned Wolff, published in 1773 : " Patet, 

 igitur, monstra composita non sic oriri, ut 



