One of the most interesting departments of embryology is that which treats of twin development, and the many forms of abnormality or monstrosity produced by the arrest or modification of its process.

When the first cleft of the ovum is incomplete at some part, and two organisms arise from its halves, arrested twin development in its widest sense occurs, and a form of double monstrosity results. That the condition is due to imperfect cleavage, and not to a fusion of two ova, is shown by the fact that the sex of the twins is invariably the same, and, as Ahlfeld has pointed out, that they are always united at identical parts. The individuals are rarely of equal size and perfection of structure, although there are several instances on record where such was the case; more commonly one of the organisms has its growth arrested at an early period of life, and clings as a parasite to the fully developed autosite. In such cases the parasite varies in size and form, existing sometimes as an almost complete organism, possessing not only external parts, but internal organs; at other times, it is represented by a limb or part of such, only (as in the case described by Handyside). This arrested development of the second organism (parasitism) is undoubtedly due to an incomplete blood supply passing to the second fœtus, a condition brought about either by placental changes, or changes within the body of the monster itself, at a very early period of intrauterine life.