Monozygotic Twinning in Mice.

Because of the predominance of multiple ovulation in mice, it has been generally accepted that evidence for monozygotic twinning would be difficult to obtain. The two attempts that have been made have failed to find any evidence for twins surviving birth at a measurable rate. In Fisher and Mather's (I936) sevenfold backcross experiment there is no evidence of twinning at all. In each of Bateman's (I960) two cases of uniovular twinning (found by dissection) one twin is so placed that it is unlikely to survive until birth. The possibility of other kinds of monozygotic twinning, and that their frequency varies from stock to stock, remains open. The present paper gives an account of one stock in which the evidence indicates a twinning rate, measured at 3 weeks of age, of about i in I00 births.