CONDITIONS OF GAMETE FORMATION 383 



even die of old age. But the more readily agamic reproduction 

 occurs in consequence of either internal or external factors, the less 

 likely is the Hfe history of the individual to attain its later stages. 

 In the case of the protozoa the question of progressive race 

 senescence has occupied the minds of most investigators to the 

 exclusion of individual senescence. Apparently, however, the 

 solution of the whole problem is to be found in the relation between 

 individual senescence and rejuvenescence under different conditions. 

 If progressive senescence in a race, ending in conjugation or death, 

 does not occur in a race bred agamically, it is not because the 

 individuals do not undergo senescence, but rather because the 

 cytoplasmic senescence in each generation is compensated by 

 rejuvenescence in each agamic reproduction and because senescence 

 of the meganucleus is compensated by the process of nuclear reor- 

 ganization which Woodruff and Erdmann have called endomixis. 



CONDITIONS OF GAMETE FORMATION IN THE MULTICELLULAR ANIMALS 



Our knowledge concerning the physiological conditions which de- 

 termine the formation of gametes in the lower multicellular animals 

 is as yet very fragmentary. As regards many forms we do not even 

 know whether sexual maturity occurs once or periodically in the 

 life cycle, or whether its appearance is merely a reaction to external 

 conditions. Observation of the animals in nature seems to indicate 

 clearly enough that in general the formation of gametes occurs only 

 when a period of vegetative growth, with or without agamic repro- 

 duction, is approaching or has reached its end. In the case of the 

 fresh-water hydra considerable experimental work has been done,^ 

 and most authors agree that low temperature determines sexual 

 maturity, although different species appear to differ to some extent 

 as regards the effective temperatures. Nussbaum maintains that 

 starvation or at least decrease in nutrition is the essential factor, 

 but other authors do not agree with him. These results do not 

 afford us any very deep insight into the physiology of gamete 

 formation. They merely indicate that a relatively low rate of 

 metabolism favors or even determines gamete formation, but 

 whether gamete formation ever occurs without the aid of external 



' R. Hertwig, '06; Krapfenbauer, '08; Frischolz, '09; Nussbaum, '09; Koch, '11. 



