THE PROTOZOAN LIFE CYCLE. 243 



periods of "depression" the race under cultivation would have 

 died out entirely, had not stimuli in the form of extracts of dif- 

 ferent substances (beef, pancreas, brain, etc.) been applied. With 

 the aid of such restoratives on three different occasions, the race 

 was carried through four "cycles" of activity and through 742 

 generations. 



There is no doubt that the organisms would have died of a 

 real physiological exhaustion, had they not been artificially stimu- 

 lated, and even when the race finally ran out, it was not from 

 physiological exhaustion in the same sense, for stimuli had again 

 been successful in restoring the vegetative functions. It was due 

 to some more deeply lying trouble, and the results confirmed 

 Hertwig's view of "germinal " death as contrasted with " physio- 

 logical ' death, and, as I have elsewhere pointed out, they 

 demonstrate that in the protozoon as in the metazoon we may 

 distinguish between somatic and germinal protoplasm. 



We can readily understand how such periods of depression 

 may be overcome in nature and stimulation effected by changes 

 in the immediate environment or, in parasitic forms, by changes 

 in the blood, and continued activity of certain parasites or appear- 

 ance of the recidive in malaria, etc., may thus be accounted for. 

 But the organisms themselves have an efficient means of bringing 

 about this renewal of vitality. It seems probable that some 

 original supply of physiological energy is continually drawn upon 

 by the vegetative organisms, some " potential of vitality," which, 

 like the charge of a battery, may become exhausted. As in a 

 battery this potential can be renewed by artificial means, but 

 unlike a battery it can also charge itself by the process of par- 

 thenogenesis. This has been shown by Schaudinn to take place 

 in the malaria organism, Plasmodiuin vivax and in Trypanosoiua 

 noctnce of the owl. Here, as in some insects, rejuvenescence is 

 brought about by the union of the kinetonucleus and the vegetative 

 nucleus, and is quite analogous to the fertilization in some meta- 

 zoa of the egg nucleus by a polar body nucleus. Protozoan 

 organisms which are thus restored by parthenogenesis seem to 

 carry with them at least one extra charge of vitality, and we have 

 no basis for speculating as to the length of time that such par- 

 thenogenetic processes may continue in a race. In the experi- 



