In 1902, Boveri introduced the important concept that for the success of fertilization the gametes must activate one another. Based primarily on studies on the sea urchin and ascidian fertilization the suggestion is presented here that "activation" of the spermatozoon actually involves a switching off of its metabolic machinery as a result of its interaction with the sperm receptors of the egg envelopes and prior to its fusion with the egg. Concerning the activation of the egg, there is a fairly large body of old and new experimental evidence that activation per se does not require sperm incorporation. Indeed, the chain of reactions culminating in the activation of the egg is initiated upon attachment of the spermatozoon to the egg plasma membrane.