394 SENESCENCE AND REJUVENESCENCE 



retard, or reverse the life cycle of the whole animal and so affect the 

 character of the eggs, or else they alter conditions in the ovaries 

 so that eggs are isolated from the parent organism earher or later 

 in their development. 



If the external conditions decrease the general metabohsm, they 

 may bring about physiological conditions which would arise without 

 their action in more advanced stages of senescence, but if their 

 eflfect is to increase metabolism, they may make the animal some- 

 what younger physiologically by increasing breakdown and elimi- 

 nation, or they may at least retard or inhibit senescence. In this 

 manner the character of the eggs may be influenced through the 

 physiological condition of the whole animal. 



It is probable, however that the physiological age and condition 

 of the egg do not necessarily correspond in all cases to the physio- 

 logical age of the egg-producing organism. Under certain conditions, 

 such as abundant nutrition or high temperature, the development of 

 successive eggs may be so rapid that each egg is forced down the 

 ovarian tubules and isolated before its growth is completed, even 

 though the animal itself is physiologically old. Such an egg must 

 be physiologically younger than one which undergoes more growth 

 before isolation. Probably the action of external factors in deter- 

 mining parthenogenesis and bisexuahty is sometimes of this charac- 

 ter, and a high rate of egg production results in younger eggs, a 

 low rate of egg production in older eggs. 



Summing up, this point of view seems to afford a basis for 

 reconcihng the apparently conflicting data, and for further analytic 

 investigation. The parthenogenic egg in the daphnids and rotifers 

 is apparently physiologically younger and less highly differentiated 

 than the zygogenic; the physiological age, both of the individual 

 and of the race, and probably also the rate and conditions of egg 

 production, are factors in determining whether parthenogenic or 

 zygogenic eggs shall be produced; and, finally, external factors act 

 by accelerating, retarding, or reversing the course of the life history 

 in the individual or race, or by influencing the rate and other 

 conditions of egg production in the ovary. 



It seems to be definitely determined that among the bees the 

 males arise from parthenogenic eggs, the females from fertilized 



