[Monozygotic heterocaryotic twinning (author's transl)].

Since the first publication in 1961, cases of monozygotic heterocaryotic twinning have been repeatedly found and a total of 14 observations can now be analysed. The mechanism involved in this type of twinning is yet uncertain and eventually is not identical in each case, the main uncertainty being to decide whether the chromosomal error affecting one of the twins is related, directly or indirectly, to the process of MZ twinning per se. Regarding the time of occurrence, the error seems to occur at few days of development at the most and, in one case at least, was contemporary to the first division cleavage. Considering the possibility of twins of different sex (e.g., one XY and one XO) the MZ heterocaryotic twinning could be considered as a potential equivalent of autofecundation in species in which the XO is a fertile female. Evolutive implications shall be discussed.