A possible mechanism of partial twinning in a calf.

A two-headed calf had doubled heads and necks, externally normal forelimbs and thorax, incompletely doubled hearts and lungs, and a persistent sinus venosus, with abnormal pulmonary and systemic circulation. Study of the specimen indicated that partial twinning involved the development of the notochord as an anteriorly branched structure, with retention of the single condition posteriorly. It is proposed that anteroposterior compression of the embryonic disk could have induced the formation of double notochords. The way in which compression was produced is suggested.