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 of two individuals in one body. There can be no question of a literal fusion of two embryos; either the individuality of each was at no time complete, or, if there were two distinct primitive traces, the uni-axial type was approximately reverted to in the process of development, as in the formation of the abdominal and thoracic viscera, limbs, pelvis or head. Double monsters are divided in the first instance into those in which the doubling is symmetrical and equal on the two sides, and those in which a small or fragmentary foetus is attached to or enclosed in a foetus of average development—the latter class being the so-called cases of “parasitism.”

Symmetrical Double Monsters are subdivided according to the part or region of the body where the union or fusion exists—head, thorax, umbilicus or pelvis. One of the simplest cases is a Janus head upon a single body, or there may be two pairs of arms with the two faces. Again, there may be one head with two necks and two complete trunks and pairs of extremities. Two distinct heads (with more or less of neck) may surmount a single trunk, broad at the shoulders but with only one pair of arms. The fusion, again, may be from the middle of the thorax downwards, giving two heads and two pairs of shoulders and arms, but only one trunk and one pair of legs. In another variety, the body may be double down to the waist, but the pelvis and lower limbs single. The degree of union in the region of the head, abdomen or pelvis may be so slight as to permit of two distinct organs or sets of organs in the respective cavities, or so great as to have the viscera in common; and there is hardly ever an intermediate condition between those extremes. Thus, in the Janus head there may be two brains, or only one brain. The Siamese twins are an instance of union at the umbilical region, with the viscera distinct in every respect except a slight vascular anastomosis and a common process of peritoneum; but it is more usual for union in that region to be more extensive, and to entail a single set of abdominal and thoracic viscera. The pelvis is one of the commonest regions for double monsters to be joined at, and, as in the head and abdomen, the junction may be slight or total. The Hungarian sisters Helena and Judith (1701–1723) were joined at the sacrum, but had the pelvic cavity and pelvic organs separate; the same condition obtained in the South Carolina Negresses Millie and Christina, known as the “two-headed nightingale,” and in the Bohemian sisters Rosalie and Josepha. More usually the union in the pelvic region is complete, and produces the most fantastic shapes of two trunks (each with head and arms) joining below at various angles, and with three or four lower limbs extending from the region of fusion, sometimes in a lateral direction, sometimes downwards. A very curious kind of double monster is produced by two otherwise distinct foetuses joining at the crown of the head and keeping the axis of their bodies in a line. It is only in rare instances that double monsters survive their birth, and the preserved specimens of them are mostly of foetal size.

Unequal Double Monsters, Foetus in Foetu.—There are some well-authenticated instances of this most curious of all anomalies. The most celebrated of these parasite-bearing monsters was a Genoese, Lazarus Johannes Baptista Colloredo, born in 1716, who was figured as a child by Licetus, and again by Bartholinus at the age of twenty-eight as a young man of average stature. The parasite adhered to the lower end of his breast-bone, and was a tolerably well-formed child, wanting only one leg; it breathed, slept at intervals, and moved its body, but it had no separate nutritive functions. The parasite is more apt to be a miniature acardiac and acephalous fragment, as in the case of the one borne in front of the abdomen of a Chinaman figured by I. Geoffroy St Hilaire. Sometimes the parasite is contained in a pouch under the skin of the abdominal wall, and in another class (of which there is a specimen in the Hunterian Museum) it has actually been included, by the closure of the ventral laminae, within the abdominal cavity of the foetus—a true foetus in foetu. Shapeless parasitic fragments containing masses of bone, cartilage and other tissue are found also in the space behind the breast-bone (mediastinal teratoma), or growing from the base of the skull and protruding through the mouth (“epignathous teratoma,” appearing to be seated on the jaw), and, most frequently of all, attached to the sacrum. These last pass by a most interesting transition into common forms of congenital. sacral tumours (which may be of enormous size), consisting mainly of one kind of tissue having its physiological type in the curious gland-like body (coccygeal gland) in which the middle sacral artery comes to an end. The congenital sacral tumours have a tendency to become cystic, and they are probably related to the more perfect congenital cysts of the neck region, where there is another minute gland-like body of the same nature as the coccygeal at the point of bifurcation of the common carotid artery. Other tumours of the body, especially certain of the sarcomatous class, may be regarded from the point of view of monstra per excessum; but such cases suggest not so much a question of aberrant development within the blastoderm as of the indwelling spontaneity of a single post-embryonic tissue. (See  .)

Monstrosities in man and animals have attracted attention since the earliest times, and amongst primitive and uncivilized peoples have been regarded as of supernatural origin. Aristotle himself appears to have been the first to examine them as a naturalist, and to explain that although they were outside the usual course of nature they were in the strictest sense of natural origin. Pliny described many well-known forms, but did not distinguish between legendary and actual monstrosities. In the middle ages they were treated in the fullest spirit of superstition, and many relics from such a point of view still survive. The human monstrosities were regarded as having been engendered in women by the devil who had commerce with them either in his own form or in the guise of some animal. The belief still to be found amongst uneducated persons that unnatural union between women and male animals, or between men and female animals, may be fertile and produce monsters, is an attenuated form of the satanic legend. The scientific appreciation of monsters has grown with the study of embryology. William Harvey in Exercitationes de generatione animalium (1651) first referred monstrosities to their proper place as abnormalities in embryonic reproduction. The doctrine of pre-formation (see ) obsessed biological science until 1759 when C. F. Wolff overthrew it, and Harvey’s advance was not pursued, except that a number of anatomists published careful studies and descriptions of monsters or monstrous organs. Those who believed that the normal process of development was an unrolling and expansion of a pre-formed miniature of the adult had to apply a similar theory to monsters, and Sylvain Regis, a contemporary of Malbranche, obtained acceptance of his view that monstrous germs as well as normal germs had been created at the beginning of the world. A discussion almost as memorable as that between E. G. St Hilaire and Cuvier on specific types was pursued in the French Academy from 1724 to 1743, J. B. Winslow, who supported the current pre-formationist view, having the better of the argument with Louis Lémery, who was almost alone in a rational interpretation of monstrosities. From the time of Wolff it was accepted that normal and abnormal embryos alike developed by processes of epigenetic change. Wolff himself, however, and even J. F. Meckel at the beginning of the 19th century, did not recognize the influence of physiological causes in the production of abnormalities; they believed the latter to proceed certainly in an orderly and natural way, but from abnormal ova. E. G. St Hilaire was the first to attempt experimental teratology and to lay down that many monstrosities were the result of influences causing deviations from the normal course of embryonic development. I. G. St Hilaire, the son of E. G. St Hilaire, carried the experimental method little further, but published an elaborate descriptive treatise on anomalies (Paris, 1832–1837) which remains one of the most valuable records of the subject. A similar treatise with an incomparable atlas of illustrations was issued by W. Vrolik, the great Dutch anatomist, between 1840 and 1849, whilst A. Forster issued in 1861 a valuable textbook with a very large number of illustrations chiefly from preparations in the museum at