Duplicate Twins and Double Monsters

DUPLICATE TWINS AND DOUBLE MONSTERS. BY HARRIS HAWTHORNE WILDER, PH. D. From the Zoological Laboratory of Smith College. WITH 2 PLATESAND 11 FIQURESIN THE TEXT. Having recently called attention to the great similarity in the config- uration af the epidermic ridges on the palms and soles of identical twins,’ and seeing that the subject is one involving important biological problems, it has seemed to me of importance to collect as much evidence BS possible on this head, and place it in convenient form, that it may serve as a basis for future speculation. Furthermore, as identical or dupZicate twins have not been generally defined save by the somewhat untrustworthy criterion of facial resemblance, and as their close relstionship to certain of the types included under the head of double monstem has not been clearly emphasized, I have begun the paper with a discussion of the general subject. This portion of the paper, which presents a series of the most important data concerning twins and compound monsters, will serve as a necessary back- ground for the facts presented in Part 11, which constitutes the more original part of the investigation. Part I11 presents the deductions as far as they seem indicated, but is intended more as an aid in directing speculation in the future than as a set of dogmatic assertions which would be at present premature. PARTI. MULTIPLE BIRTHS AND THEIR RELATIONSHIP TO COMPOSITE MONSTERS. DUPLICATEAND FRATERNALTwINs.-It is well known that there are, at least in the human species, two types of twins ; the first include those cases where the sex may or may not be the same and where the general resemblance is about what may be expected in the case of any two children lCf. Palms and Soles, in Smer. Jour. Anat., Vol. I, p. 423, Nov., 1902. AMERICANJOURNAL OF ANATOMY.-~OL.111. 388 Duplicate Twins and Double Monsters of the same family; the second, those who are invariably of the same sex and who otherwise so closely resemble one another that it is difficult or impossible, especially during youth, for those not intimately associated with them to distinguish between them, the so-called ‘‘ identical ’’ or “ homologous ” twins. Although these two types are both very common, the Gecond rather more than the first, there seems to be in the popular mind no clear dis- tinction between them. That there is a general impression that twins ought to look alike appears from the emphasis placed upon cases where they do not, but that this identity of facial expression does not extend to twins of opposite sex is a fact not commonly apprehended, and instances in literature are not rare in which a young woman in disguise is passed off as her twin brother, or the reverse. As a matter of fact all twins of opposite sex, as well as many cases in which the sex is the same, belong to the first, or non-identical type, while for the second type an identity of sex as well as of facial expression and other bodily peculiari- ties is a prereguisite. Concerning the nature of this peculiarity, the (most plausible and, in fact, the only hypothesis is that twins of the identical or homologous type are produced by the division of a single fertilized egg, while the other type results from the fertilization of two separate eggs, either from the same or different ovaries, and are thus two fundamentally distinct indi- viduals, i. e., a case of multiple birth such as normally takes place in most species of mammals. As expressed recently by Weismann, 02 (11, p. 54), “ Wir haben nun allen Grund, (lie erste Art von Zwillingen (i. e., fraternal) von zwei verschiedenen Eizellen abzuleiten, die letztere Art aber (i. e., duplicate) von einer einztgerz, welche erst nach. der Befruchtung durch cine Samenzelle sich in smei Eier getheilt hat.” * Corresponding 21t seems impossible, with any degree of certainty, to place the credit for the first enunciation of the above hypothesis. Although often attributed to Francis Galton, Baudouin, 91 (p. 274), ascribes it to Camille Dareste, the noted teratologist, who in 1874 defended this theory before the Societe d’Anthropologie against the opposition of Paul Broca. Fisher, however, in 1866, antedating the statements of either of the above on the subject, advances the same hypothesis to account for the formation of double monsters, stating that they “ are invariably the product of a single ovum, with a single vitellus and vitelline membrane, upon which a double cicatricula, or two primitive traces, are developed” (66, p. 208). As Fisher published in a magazine not readily accessible, at least at that time, to foreign investigation (Trans. Med. SOC.State of N. Y.), and as the similarity of separate and united duplicates might not have appealed to them, the formulations of both Galton and Dareste may well have been arrived at independently of Fisher’s theory, and the same ideas may have occurred also to others working in the same field, since the hypothesis is of so obvious a nature. Harris Hawthorne Wilder 3s9 to this hypothesis, which, in the light of our present knowledge, appears to be not far from the truth, we may designate these two types respectively as Fratemk and Duplicate, thus doing away with the misleading and in- applicable terms “ identical ” and “ homologous ’’ as applied to the one type, and furnishing a distinguishing term for the other, which seems thus far to have remained without a name.’ INTU-UTERINERELATIONS.-AS the discussion of the origin of these two types of twins leads us to the consideration of the conditions which obtain during early embryonic life, we naturally turn to the observations furnished by obstetricians ; but this source, although supplying numerous illuminating facts, is less valuable than it should be, owing to the fact that medical men share the popular confusion noted above in regard to bwins and that, while they record trushorthy details concerning placen- tation and other relationships, they fail to correlate with these the necessary data concerning sex and general resemblance, the last item of which involves the following up of the case through several years of develop- ment, a line of work hard to accomplish during active professional life. The most noteworthy set of data covering these points are those tabulated by 0. Sahultze, 97, who gives in the form of a classification the various intra-uterine relationships which have been observed in twin births, with suggested correlations of the type of twin produced in each case. As this table is so essential to the present inquiry, I will transfer it in a somewhat abbreviated form, modifying its very accurate terminology to con- form to that in more general use.4 INTRA-UTERINE RELA4TIONSHIPSIN TWINGESTATIONS. Case I.-Two separate blastodermic vesicles with two deciduae reflex= and two placenta This case is probably one in which there are two separate eggs, either from the same or from opposite oviducts, and implanted *Strictly speaking, the word “ fraternal ” applies only to twins of the male sex, since in Latin, as well as in English, there is no word which, like the German “ Geschwister,” applies to sisters and brothers alike. The present use of the word in question, however, corresponds to that of the English masculine pronoun “ he ” in similar cases, and thus seems entirely warrant- able. Pearson’s term ‘‘ Sibling ” is correct in meaning, but is so rarely used that I hesitate to employ it. Schultze confines the term “ Keimblase ” (blastodermic vesicle) to the blastula stage of the embryo, employing for the later stages, to which the same term has been generally applied hitherto, the term “ Fruchtblase.” The parts surrounding this and supplied by the uterine mucous membrane are termed collectively “ Fruchtkapsel,” the free portion of which is the decidua capsularis (decidua reflexa autt.) . 390 Duplicate Twins and Double Monsters at some little distance from one another. In one case investigated by v. Kolliker, the two deciduae were distinct but partially adherent over the surfaces in mutual contact, and in another the contact surfaces had fused into a single wall into which, from the two opposite sides, the chorionic villi of the two embryos had grown. In addition to this, one of the placents was of the typs known as a placenta. marginata, caused by a fold of the decidua. [This case is evidently a normal multiple birth, a condition hard to accomplish in a uterus of the shape found in human beings, and often attended by such phenomena as adhesions, fusions and foldings, all indicative of crowding and of nothing else.] Case 11.-Two separate blastodermic vesicles enclosed in a single decidua. Placentze fused with one another but with two separate sets of umbilical vessels. Two chorions, fused at the point of contact. This case is more frequent than (I) but apparently results from the same general cause, i. e., two separate eggs, which are, however, implanted nearer together. This would seem more likely to happen if both eggs came from the same side. [The conditions are seen to be similar to those of (I), the greater degree of fusion being well accounted for by the greater approximation of the two eggs to one another.] Case II1.-Two amnions and two umbilical cords but with a single placenta, in the middle of which the two cords meet and upon which the umbilical vessels closely anastomose. These are enclosed in a single chorion and covered by a single decidua reflexa. This case is said by Hyrtl to be more frequent than (I) and (11) but is not as frequent according to Xpath. The twins are always of the same sex.

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