MORE COMPLICATED LIFE CYCLES 127 



and female protozoan individuals. The evidence for this conclusion 

 is in every case somewhat inconclusive ; the differences seemingly are 

 not beyond the range of individual variation. 



In the majority of free forms, gamete formation, with their libera- 

 tion, is accomplished in the ordinary medium in which the organisms 

 live, although these processes may be hastened or influenced by 

 artificial changes in the environment. Thus, Hertwig ('98) noted that 

 the quantity of food had much to do with these phenomena in the case 

 of actinospherium, and Klebs, Dangeard, Greeley, and others have 

 found that changes in temperature or in density of the medium may 

 induce gamete formation in different kinds of flagellates. Similar 

 changes in environment seem to be a sme qua non for sex differentia- 

 tion in many parasitic forms, the most notable and best-established 

 case being the malaria organisms where microgametes are formed only 

 in room temperature, in the mosquito's gut, or, in general, in a colder 

 (denser?) medium than the blood. 



In the great majority of cases where gametic differentiation obtains, 

 if the gametes do not conjugate they die. This is invariably true of 

 the microgametes, and their fate is probably due to the extreme 

 specialization which they have undergone. In the female forms this 

 is not the invariable fate, for in some cases the cells undergo partheno- 

 genesis, a process of renewal which is accompanied by nuclear activi- 

 ties of a special kind. (See Chapter IV.) 



C. The Period of Old Age. — Protozoa quickly die after the period 

 of maturity is passed, and old age, the final period of a life cycle, is 

 rarely seen or recognized. Maupas ('89), however, using the culture 

 method, gave a very graphic description of old age in certain forms 

 of infusoria. Thus, in Onychodr omits grandis the body of the cell 

 becomes much reduced in size, loses cilia and cirri, while other organs 

 both external and internal, atrophy, and the organisms die of senile 

 exhaustion. In Paramecium aurelia the circumstances accompanying 

 old age have been described above, but in this case the metabolic 

 processes had been restimulated, and apparently the cell organs were 

 suitable for a continued activity, but something was wrong and the 

 race died. This "something" had to do with the germ plasm, for, 

 as stated, the micronucleus was hypertrophied and divisions were 

 abnormal. 



"The first clearly marked period of depression came in July, about 

 six months after the cultures were started. It was characterized 

 by a well-defined reduction in size (down to 109 microns) and by 

 vacuolization of the endoplasm, while the ectoplasm did not appear 

 to be much involved. iVIany of the individuals were characterized 

 by great vacuoles similar to those in starved forms, which dis- 

 torted the body almost out of recognition; in others the nuclei were 

 frao-mented into two or three parts, and in all there was a marked 



