134 PROTOPLASMIC AGE OF PROTOZOA 



germinal elements. It is to be compared with the germ plasm con- 

 tained in the germ glands of the many-celled animals, while the macro- 

 nucleus and the cytoplasm are to be compared with the relatively 

 much more voluminous somatic tissue of the higher animals. Its 

 degeneration, therefore, indicates an exhaustion of the potential of 

 activity of the germinal functions, including the power to divide, and 

 with this exhaustion comes the death of the race, but death due to 

 "germinal" rather than physiological exhaustion. While physiological 

 death may be averted by stimulants of different kinds, germinal death, 

 at least in the experience of all investigators up to the present time, 

 cannot be offset, and with this comes the inevitable death of the race 

 of protoplasm or death from germinal old age. Still paramecium are 

 plentiful in ditches and ponds, a fact indicating that there is some 

 natural way in which germinal death can be averted. Here is where 

 the process of fertilization comes into play, and with fertilization the 

 protoplasm of an exhausted paramecium is made over into a new 

 "individual," in the same way that the protoplasm of a germ cell of a 

 bird, mammal, or man is made over into a new individual. 



These various experiments indicate, therefore, that natural death 

 from old age under the conditions of the laboratory is actually inherent 

 in protoplasm as little differentiated as in these single-celled animals, 

 and they fail to confirm Weismann's claim that natural death is a 

 penalty which higher animals must pay for the privileges of differenti- 

 ation. They likewise fail to show that natural death by old age is due 

 to any malevolent action on the part of certain structures of the body, 

 as Metchnikoff would have us believe is one cause of old age in man. 

 It is a natural condition of all protoplasm to grow old, and if we find 

 the phenomenon in the generalized cells of the infusoria, how much 

 more probable is it in the highly specialized somatic cells of the body. 

 Each Paramecium has a certain allotment of natural life and division. 

 I have called it the potential of vitality. When this is exhausted under 

 given conditions the protoplasm dies. It also has a certain allotment 

 of germ plasm, so that by exhaustion of the physiological potential 

 it still may retain a certain capacity for cell division, the germinal 

 potential not being exhausted. It may, therefore, be stimulated by 

 artificial means. In different kinds of animals and in different indi- 

 viduals of the same species it is probable that the initial potential 

 varies, in some representing a longer, in others a shorter, life. In 

 Paramecium and the protozoa generally we find the greatest relative 

 germinal potential, but as we go higher in the animal scale the ten- 

 dency is for the germinal plasm to concentrate in a definite tissue of 

 cells, the germinal epithelium, while the somatic cells have a corre- 

 spondingly low degree of germinal plasm. To illustrate, while in all 

 probability every cell of the paramecium race is capable of becoming 

 or of giving rise to a germ cell, the same is not true of the animals next 



