The ovulated eggs are exogenously treated, following the natural collection thereof, with at least one signal transduction molecule naturally occurring in the egg cell, for example hydrogen peroxide and/or calcium chloride. In this way, the ovoperoxidase is activated, causing the formation and hardening of an extracellular egg membrane. The treatment of the ovulated eggs by a molecule which naturally occurs in the egg cell and is otherwise formed in the normal cell metabolism in order to prevent polyspermia enables a completely unharmful treatment possibility which is generally recognised as safe for hardening the ovulated eggs. The degree of hardening can be adjusted by the duration of the treatment.; The hardened eggs, the artificial hardening of which can be examined by a specific structure under an electron microscope, can be better preserved and stored as the method also has a bactericidal action, preventing a tyrosine crystallisation.