Q. Concerning their size and basic morphology how and why do the male and the female gametes differentiate from each other?
The female gametes are big cells full of vitellus (nutritive material). The male gametes are mobile, small and agile flagellate cells.
Those features are related to their respective biological functions while the female gametes have the essential functions of receiving the sperm cell nucleus and of storing nutrients for the zygote, the male gametes have the function of active movement towards the egg cell.