Charak Samhita
शुक्राधिकं द्वैधमुपैति बीजं यस्याः सुतौ सा सहितौ प्रसूते। रक्ताधिकं वा यदि भेदमेति द्विधा सुते सा सहिते प्रसूते ॥१३॥
When the zygote with dominance of sperm is bifurcated , the woman delivers male twins and when the one with dominance of ovum is getting bifurcated, female twins are born.
english translation
zukrAdhikaM dvaidhamupaiti bIjaM yasyAH sutau sA sahitau prasUte। raktAdhikaM vA yadi bhedameti dvidhA sute sA sahite prasUte ॥13॥
hk transliteration by Sanscript