Again, after hibernation regeneration is often extensive. The large proportion of young cells in the body in such cases renders the animal as a whole appreciably younger than at the beginning of hibernation, so that the periodic cycle of activity and hibernation is much like an age cycle. This rejuvenescence may begin during the hibernation, when the animal is living on its own substance. Again, we see periodic changes that resemble the age cycle in glands. In the pancreas cell, for example, the loading of the cell is both morphologically and physiologically similar to senescence, and the discharge, to rejuvenescence. In this case the change occurs in individual cells without cell reproduction. Even the cells of the nervous system throughout mature life possess no appreciable capacity for differentiation and regeneration beyond the power to regenerate fibers arising from them. Child believes that the effect of a change in mental occupation or of a vacation may afford “some slight degree of rejuvenescence of the nerve cells.” Verworn, he tells us, distinguishes between fatigue due to accumulations that check metabolism and exhaustion due to lack of oxygen, both of which may cause senility in nerve cells. “Thus exhaustion resembles senility as death from asphyxiation resembles death from old age.” Recovery from exhaustion is not the same sort of change as rejuvenescence except as it involves increase in the rate of oxidization. But fatigue and recovery constitute a cycle resembling closely the age cycle.