104 PROTOPLASMIC AGE OF PROTOZOA 



species as those of the younger generations. It is for this reason, 

 mainly, that in recent years a number of biologists have strongly 

 advocated the use of the entire life cycle of a protozoon rather than the 

 cell , or many cells in the same stage of vitality, for the basis of species. 



While Biitsehli ('76) was the first to note the differences in vitality 

 in a race of protozoa, and Hertwig, Maupas, and a score of others 

 added many observations on different periods, it was Schaudinn 

 (1900) who first clearly perceived the importance of studying the com- 

 plete life history of every species. It is because of this importance that 

 the life cycle forms such a conspicuous part of the definition of protozoa 

 as given at the beginning of Chapter I. 



Before outlining a typical protozoon's life history, it will be necessary 

 to understand clearly what is meant by age in protoplasm. It is quite 

 evident, broadly speaking, that there is some protoplasm that does not 

 die, the living things on the earth today testify to that, for they repre- 

 sent protoplasm that has been continuously living since the advent of 

 life on the earth, and which, through posterity, will continue for an 

 indefinite time in the future. Such protoplasm forms the substance 

 of the germ cells, and they alone of all cells have the potential of an 

 indefinite existence. But this capacity to live without finite end is 

 bound up with a biological phenomenon as little understood as life 

 itself, namely, fertilization. Without the union of two germ cells even 

 this endowed protoplasm would die no less surely than do tissue cells. 

 The protozoa are like both tissue cells and germ cells, and consist of 

 protoplasm which is differentiated into somatic and germinal parts, 

 and this protoplasm, like that in higher cells, will die of old age if 

 fertilization or its equivalent is prevented. The problem of age in 

 protozoa, then, has to do with vitality as apart from the union of germ 

 cells and as manifested in the ordinary processes of vegetative activity. 



I. A TYPICAL LIFE CYCLE. 



The manifestations of protoplasmic activity which occur in all cells 

 from monads to man, involving processes of digestion, growth, irri- 

 tability, etc., are easily studied in Paramecium aurelia, a very common 

 infusorian that may be found in any stagnant ditch or pool (Fig. 37). 

 To a trained eye it may be seen without the aid of a lens as a minute 

 white spot of protoplasm which moves from place to place in an irreg- 

 ular line of motion. When magnified it appears as an asymmetrical, 

 cigar-shaped organism, with a somewhat spirally wound depression or 

 "peristome" leading from one end toward the mouth near the centre 

 of the body. Within the protoplasm is a large nucleus, macronucleus, 

 usually ellipsoidal in form but subject to wide variations in size; and 

 a smaller nucleus, known as the micronucleus, which is embedded in 



