A malformation is any deviation from the normal embryonal development. This may be the result either of some disturbance taking place in the self-developing power of the embryo, or else due to some influence directly concerned with the maternal structures. The causes may be either internal, existing in the embryo itself, or external, those acting from without.

If the development is only slightly imperfect the condition is called a malformation; if marked, it is a monster. The defect may occur within an individual or there may be two or more united individuals, the latter being either double or triple monsters.

To have any serious malformation taking place, the causes must have begun to exert themselves very early in embryonal life. In such cases the lesions will generally be of such a nature that extra-uterine existence is impossible. It is probable that such take place before the third month.

The malformations brought about by external causes usually occur during the later period of fetal development. As a rule, they are not of sufficient gravity to prevent the child from living.

Some of the departures from normal occur in different cases, but with about the same appearances; these are spoken of as typical and are due generally to some internal cause, harelip being such an example. If the malformation is entirely unusual it is atypical, and results from external factors.

If the variation is one that is present in either parent it is spoken of as an inherited abnormality, as in the frequent occurrence of extra fingers or teeth in successive generations; is also seen in the way in which certain diseases are transmitted from parent to offspring.

If the abnormality passes over one or two generations before reappearing, the condition is known as atavism.