PROTOPLASMIC AGE OF PROTOZOA 103 



physiological changes in the unicellular animals, and can we distin- 

 guish periods of youth, maturity, and old age? 



Since the fundamental biological laws are much the same, on 

 a priori grounds alone we should expect to find the same series of 

 changes in protozoa as in the metazoa. But while we do find them 

 in protozoa, they are manifested in a way that we would not at first 

 suspect. We have been accustomed to look upon the single-celled 

 ameba, or paramecium, or other protozoon, as a complete individual 

 in itself; but when we come to compare such an individual with a 

 metazoon we do not find the analogous periods of vitality which in 

 metazoa we recognize as youth, adolescence, and age. A protozoon 

 is a free-living cell, a complete organism indeed, but as such it has 

 no period of youth nor of sexual maturity, nor, by itself, old age. It 

 is formed by division or some modification of division; it regenerates 

 the normal form in a few hours, and then again divides; with division 

 its individuality is lost, to be merged into that of two new individuals, 

 these two into four and so on. Obviously such an individual cell 

 presents nothing comparable with the sequence of stages so char- 

 acteristic of the "individual" in higher forms of life. 



Students of the protozoa and biologists generally (e. g., Btitschli, 

 Weismann, etc.) early called attention to the fact that not the single 

 cell of a protozoon, but the entire succession of cells that may be 

 formed from the period of one conjugation to that of the next, should 

 be compared with the metazoon. In the latter, the fertilized egg 

 cell gives rise to a multitude of body cells by repeated divisions; 

 the cells are bound together to form a uniform and differentiated 

 whole. In the former, the fertilized protozoon divides, but the cells 

 do not remain bound together; they separate and live as independent 

 units. If we could take such an entire succession of cells thus formed 

 from the repeated divisions of a fertilized protozoon, and if at any 

 given period could combine them in one mass of cells, we would have 

 the analogue of a metazoon and would find that the protoplasm 

 represented by the aggregate of cells would manifest the same suc- 

 cessive periods of vitality as those of youth, adolescence, and old age 

 in metazoa. We would find that the young cells divide more rapidly 

 than they do later in the cycle; we would find that after a certain 

 period they become sexually mature and able to conjugate and so to 

 perpetuate the race; and we would find that, ultimately, evidences of 

 weakened vitality and degeneration appear in the aggregate of cells, 

 and that they would finally die of old age. 



Not only would such an aggregate show the characteristic periods 

 of vitality, but with the changes from one period to another there 

 would be, in a great number of cases, accompanying changes in the 

 form of the cell body; changes of so great a nature that a casual 

 observer would never regard such cells as belonging to the same 



