MONSTROSITTKS. 273 



As already observed, tliere are donbtUss confrcnilal or lieredKary causes 

 oecasioniiin- luonstrosiiies, and wbieb latter may bo divided into double and siu<;\o 

 ones. The chief hypotheses whieli have been oiTered to account foi-(loubU" mon- 

 sters arc (1) by /W/oji, or that they liavc been formed fi'om two distinct end)ryos 

 Avhieh have become united or fused together. (2) By Jlssion, or that they havo 

 sprung from a single germ, whieli has doubled or become sub-divided. (3) That the 

 germ itself was abnormally compound from the very first. 



It has been observed that we may arrange these monsters as existing in the 

 higher forms of vertebrate animals in a continuous series from such as possess an 

 extra finger or too, to those in which two or oven three heads have been present. 

 While the examination of a large number of specimens has led to tlie conclusion 

 that superficial portions of animals are more liable to multiplication than arc 

 internal organs, and those of the upper or anterior half of a body than its inferior 

 or posterior extremities.* 



If, however, ^\c restrict ourselves more to fishesf we can observe monsters with 

 three heads (plate xii, fig. 11 and 11a), or with two heads (plate xii, fig. 8), or 

 the chest may likewise be doubled ; or twin fish completely developed, but pos- 

 sessing a single yelk-sac (plate xii, fig. 10) ; or only one is completely developed, 

 the other being more or less in a rudimentary condition (plate xn, fig. G). Or 

 monsters with three or two heads may only possess one tail (plate xii, fig. 11), 

 or they may be united by their tail portion (plate xii. fig. 9). Or the head and 

 body may be single anteriorly, but the tail portion may be double (plate xu, fig. 

 4). Or there may be three eyes (plate xii. fig. 1), or even four, and these may be 

 variously situated. 



M. de Quatrefagest considered, An7ials of Natural History,^ xv, 1885, p. 47, 



* Among vertebrates double monsters have been thus divided :— 1. Anterior ditpUcity, when 

 two bodies become adherent to one another by their anterior surfaces (as by the sterna). 2. 

 Lateral diq)licity, as a common thoracic cavity : or it may be in two principal divisions, as 

 duphcity of the entire body, terminating in singleness, or duplicity of the remammg entire body, but 

 the head continues single. Or in some cases the two heads begin to coalesce: then only one car 

 remains between the adjacent surfaces of the two heads ; or both ears may become lost ; or the 

 two adjacent and middle eyes approximate, next there may be only one orbit ; or union of tlie 

 heads • or the head merely doubled in individual parts. The body may be smgle in the middle, but 

 double above and below; or the body may be single above and double below. 3. Lijerwr 

 dupUcity, or two bodies with their lower ends united, a head above and another below. 4. F0!<- 

 terior duplicity, two bodies united by their backs, or portions of them. 5. Superior duphnty, as 

 two children which have been born connected by their skulls. It has been remarked that there 

 has been only one triple human monster recorded. 



t See M. Girdwoyn, PatJioloyie des Poi.'isom, 1880. . 



t A commonly expressed error is that double-yolked fowls' eggs always contain two embryos, 

 and that during incubation one generally develops to the partial or entire destruction ot the 

 other, and that thus extra heads or organs are produced. Thompson, however, LoMon and 

 Kdinhurqh Monthly Journal, July, 1844, tried to hatch examples of these eggs but failed ; in 

 some it was evident that only one yolk was productive, and it would appear that double monsters 

 are not dependent upon double yolks, although it has been thought that from such possibly twms 

 might be produced. . ,, , ,^,, ,„-, i ii 



8 M de Quatrefages exhibited at the French Academy of Sciences, March 19th 18o4, a double 

 monster which he kept alive nearly two months; it consisted of two fishes completely separated 

 one from the other, and adhering to the opposite sides of a vitellus, which showed a deep notch 

 in the front. Of these two fish the largest had its face deformed, its eyes wei^ absent but the 

 remainder of its body was perfect. The second or smaller fish liad its head well formed, but its 

 body was humped and its tail twisted. The abdominal veins (afterwards converted into the rence 

 vortic) were in their normal situation, their ramifications spread over tlie whole surface ot the 

 vitellus, communicating at their extremities with the roots of the vitelline veins, which sub- 

 sequently form the hepatic veins. Also numerous anastomoses connected the last ramiiications 

 of the abdominal vein of each embryo with those of the vitelline vein of the other, so that a 

 continual interchange of blood took place. _ , , ^ , , , 



On February I'Jth, nearly a month after the specimen came into M. Quatrefages possession, 

 and about six weeks after exclusion from the egg, the two embryos were close together, and reac y 

 to unite on one side of the abdomen, while on the other they were still separated by a considerable 

 space occupied by the vitellus. The larger embryo had originally been situated to the right of 

 the vitellus, but had become superior, lying somewhat across the smaller and more deformed 

 individual, which it carried about with it. . -^ i. i u t^,.^.r.A y.-^ 



M. de Quatrefages, as well as M. Serres, concluded that this monstrosity had been foimed by 

 the coalescence or fusion of two originally distinct embryos, and that the vitellus from which it 



io 



