CHAPTEE IV. 



CONJUGATION, .MATURATION, AND FERTILIZATION. ' 



In the preceding chapter it was shown that the protoplasm of which 

 a protozoon is composed, as demonstrated by continual observation, 

 gives evidence of advancing age no less surely than does a many- 

 celled organism. It was shown further that the advance from youth 

 to age in such protoplasm is indicated by more or less well-marked 

 physiological and structural changes, the former being characterized 

 by the onset of a noticeable "period of depression," the latter by 

 morphological changes, of which the most important is the develop- 

 ment of a well-defined germ plasm. Experimental work on free-living 

 protozoa has shown that the cells die a natural death during such 

 periods of depression, but also, in some cases, that these periods may 

 be overcome by artificial stimulatioji. They show, also, that a final 

 depression, distinguished from ordinary physiological or metabolic 

 weakness, and characterized by loss of the germinal protoplasm, could 

 not be thus overcome. Apart from death by violence, therefore, the 

 free-living protozoon may lose its life by what Hertwig calls "physio- 

 logical death" at some period of physiological depression, or by 

 "germinal death" occurring with the exhaustion of the division energy 

 and degeneration of the germ plasm. 



Notwithstanding the many natural enemies which a paramecium 

 or other j>rotozoon has, and in spite of the fact that if it escapes such 

 enemies it may die from physiological or germinal "old age," it still 

 exists in more or less abundance in natural waters, and will probably 

 continue to exist in the future. In natural waters, salts, changes in 

 the local environment, and other external causes undoubtedly tend 

 to stimulate lagging physiological activities and to do on a large scale 

 what we have done in the laboratory; but in nature, as in the labor- 

 atory, such means of rejuvenation probably have their limits, and we 

 must turn to other vital activities for an explanation of the continued 

 existence of these living cells. 



There is little reason to doubt .that the ex])lanati()n lies in the 

 secrets of the same mysterious and at present unfatlionied phenomena 

 which miderlie the newborn infant; which are repeated in all living 

 things with the creation of a new individual; and which are univer- 

 sally regarded as among the subtlest of vital activities. These 

 secrets are (leej)ly hidden in the ])henoniena of fertilization, and 

 philosophers today, like the ancients, have only speculations to od'er 



