The belief has been productive of great anxiety and even unhappiness during a period which is necessarily a trying one, and should be dismissed as being both theoretically impossible and unsupported by fact.
The malformations are divided anatomically into those characterized, first, by excess formation, second, by deficient formation, third, by abnormal displacement of parts.

They are due to intrinsic causes which are in the germ, and which may be due to some unusual conditions in either the male or female germ cell or an imperfect commingling of the germinal material, and to extrinsic causes which physically, as in the nature of a shock or chemically as by the action of a poison, may affect the embryo through the mother.

Malformations are made more numerous in chickens by shaking the eggs before brooding.

A number of malformations are produced by accidental conditions arising in the environment; for instance, the vascular cord connecting mother and child may become wound around parts constricting them or even cutting them off, and the membrane around the child may become adherent to certain parts and prevent the development of these.

The extrinsic causes are more operative the more unfavorable is the environment of the mother.