This is precisely analogous to the case of gymnosperms, lycopods, etc., where on leaf structures are formed macro and micro sporangia.

This formation of germinal vesicles and prothallium seems very different from the formation of archegonia and prothallium in Selaginella, for instance; but the link which connects the two is in the gymnosperms, where distinct archegonia in a prothallium are formed.