Monstrosity is the term applied in human and comparative anatomy to an aberrant formation of the body consequent upon early disturbances in the developmental processes in the embryo. Teratology (teras, logos), the special and very interesting branch of biology which deals with the causes of such occurrences and with the classification of the 'monsters' so produced, has been advanced by the researches of Geoffroy Saint-Hilaire, Förster, and others to the position of a special science, and one that throws a valuable sidelight on that of normal embryology. The malformations to be dealt with may affect the whole organism or portions only of its structure. Monsters are, however, usually classified under three headings: (1) Those with exaggerated or supernumerary parts (monstra per excessum); (2) those lacking parts (monstra per defectum); and (3) those with abnormally arranged parts (monstra per fabricam alienam). Those of the first class, where supernumerary limbs or a double head or trunk exist, are generally recognised as due to the more or less complete fusion of two or more embryos, originally separate, during the process of development. Cases of this kind which have from time to time been carefully described, figured, or preserved in museums show that almost every possible degree of fusion of separate embryos may occur, resulting in a correspondingly great variety in the shapes of the monsters produced. Two otherwise complete bodies may be attached by an external bond, as in the case of the Siamese twins; or the one may be wholly or partially enclosed by the tissues of the other. A case of such complete inclusion is found in the Hunterian
Museum. Much more frequently, however, but imperfect relics of the one remain attached to, or fused with, the fully-developed structures of the other. Thus arise two-headed monsters, those with double trunks or double sets of limbs, and those in which a shapeless mass representing the blighted embryo remains attached to the fully-formed body of the twin organism. In this same class of monsters by exaggeration must be placed also cases of general or local gigantic development, due not to fusion of separate embryos but to general or local precocity of growth in the tissues of a single organism. Not less interesting are monsters of the second class, where entire parts of the body may be suppressed during development. Here again it is shown that the non-development may occur in any region and to any extent: consequently numerous and widely separated varieties of monster are found in this class. The suppression of parts varies likewise in degree, and in its effect upon the viability of the organism. For instance, a headless or brainless monster is of necessity incapable of living; whereas one with suppression of a limb is viable, and might more properly be described as a case of congenital deformity. In the third class are the cases of transposition of viscera, malposition of limbs, congenital dislocations of joints, &c. See DEFORMITIES, CLUB-FOOT, and, for monstrosity in plants, TERATOLOGY.