[Anencephaly and diprosopy: 2 cases].

We have seen two cases of diprosopy associated with anencephaly in Brittany between 1975 and 1984. Diprosopy is a partial or total duplication of the face. It consists of the phenomenon of late division in the embryo of the cephalic portion of the neural plate between the 16th and the 18th days. This gives rise to an incomplete type of monozygotic twinning or a conjoint twin. There are several different forms of the organs that are duplicated. We have seen a case of diprosopos distomos dirhinos diophthalmos and a case of disprosopos distomos dirhinos triophthalmos. These two cases were associated with anencephaly, the second also having a spina bifida and a diaphragmatic hernia. One can explain the incidence of anencephalies in cases of diprosopies by the desturbance created by the latter on the embryological events that succeed it. The delay in nerve formation makes it impossible for the neural tube to close completely, and this is why sometimes the anencephaly is associated with spina bifida. In more general terms one can postulate that all conjoint twins that are, of course, monozygotic and monochorial can interfere with early enbryological development and increase the risks of failure of the neural tube to close.