You mean this :Wel, you do see the dark seam already developing, which will follow the growth of the sword to be.
In French it is said protogynous hermaphroditism (possibly different in English).Only females can turn into male. But once they are transformed, it can't go back to female.
Wow this IS incredibleXiphophorus females that went through a transition to male, will be functional males.
No one tell me fishes aren't extraordinary living-beings !