The activity to which each spirit germ tends towards determines its sex. If it tends towards a positive activity, it assumes the male form and if it tends towards the negative activity, it assumes the female form. The shape thereby forms itself.
The form is fundamental for the existence of the spirit and it cannot just change sex at will. Some people incarnate alternatively as male or female. This is not normal. It results, in the case of a male soul, a tendency towards negative activity in a particular earth life. The next incarnation will then be in a female body. The same applies to a female soul tending toward a positive activity.