MONSTROSITIES. 273 



As already observed, there are doubtless congenital or hereditary causes 

 occasioning monstrosities, and which latter may be divided into double and single 

 ones. The chief hypotheses which have been offered to account for double mon- 

 sters are (1) by /msi'ow, or that they have been formed from two distinct embryos 

 which have become united or fused together. (2) By fission, or that they have 

 sprung from a single germ, which 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 toe, to those in which two or even three heads have been present. 

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

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

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

 or posterior extremities.* 



If, however, we 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 xii, fig. 6). 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 xii, fig. 

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

 variously situated. 



M. de QuatrefagesJ considered. Annals of Natural Sistory,% xv, 1885, p. 47, 



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

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

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

 dupMcity of the entire body, terminating in singleness, or duplicity of the remaining entire body, but 

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

 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 the 

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

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

 duplicity, or two bodies with their lower ends united, a head above and another below. 4. Pos- 

 terior duplicity, two bodies united by their baots, or portions of them. 5. Superior duplicity, 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, Pathologic des Poissons, 1880. 



I 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 of the 

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

 Edinburgh 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 twins 

 might be produced. 



§ M. de Quatrefages exhibited at the French Academy of Sciences, March 19th, 1854, 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 were absent, but the 

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

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

 porta) were in their normal situation, their ramifications spread over the whole surface of 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 ramifications 

 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 19th, 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 ready 

 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. 



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

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



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