These form a very important group, and include a large proportion of the more considerable malformations of the body. They present great variety in form and degree. At the one end of the scale we have the monstrosity consisting of two complete individuals, united by a narrow band, or indeed disunited. At the other end we have simple duplicity or reduplication of parts.

In all cases of double monstrosity there has been, to begin with, one ovum. Opinions have been divided on the question whether in the single ovum there have been originally two embryoes which have partially united, or' whether there has been but one embryo which has given rise to the doubled parts by fission. The latter is the more likely explanation in most cases of double monsters.

Multiplication by fission is common in the vegetable kingdom, and is not unknown in the lower forms of animal life. Experiment seems to show, that stimulation of the early embryo may lead to doubling in whole or in part. Schultze, for instance, asserts that in the eggs of amphibia, an alteration in the position of the ovum in certain ways, produces a double malformation. We may suppose that stimulations of unknown origin may affect the embryoes of the higher animals. Thus, should there be over-stimulation of the ovum soon after impregnation, there may be fission of the whole germinal mass, with the ultimate production of two separate individuals or twins, which may, however, be partly united. At slightly later periods, when the central part of the cerebro-spinal axis has been already laid down, the fission may still affect the upper or lower end of the embryo, and we may have doubling of these either together or singly, while a portion of the cerebro-spinal axis is single. Then at still later periods when the appendages are being formed, there may be a more local fission leading to multiplication of parts. There are even cases in which the fission has been repeated more than once, so that there is not only a doubling but a trebling of parts, and the monster contains portions of three individuals.

An interesting confirmation of the view that double monstrosity arises by fission of one embryo has been furnished by Cleland in his observations on supernumerary legs. See further on at p. 39.

The doubled parts may vary in the degrees of development which they attain. The one may grow much more quickly than the other, and when that is the case the lesser may undergo various degrees of displacement and defective development. As the well-developed division grows in length it may even tear away parts of the less developed one from their relations, and so produce complex arrangements. The more quickly growing part may even come to include the less developed one, and the latter may form a kind of Parasite on the former.

Another law has to be remembered in considering double monstrosities, namely, that by which symmetrical parts tend to adhere. This law has a wide application in normal development, lateral parts adhering and coalescing in the middle line. It has also applicability in cases of doubling, the symmetrical parts of one division sometimes adhering to the symmetrical parts of the other. There is, however, not infrequently a partial coalescence and suppression of parts of double monstrosities if they meet in the middle line.

The large majority of double monstrosities are of the female sex, and the two bodies are always similar in sex.

The double monstrosities are divisible into four groups according to the extent and situation of the fission. The division is (1) complete, involving the whole cerebro-spinal axis, or (2) of the hinder extremity - abcaudal fission, or (3) of the anterior extremity - abcranial fission, or (4) of both extremities while a portion of the cerebro-spinal axis is undivided.