240 THE PROTOZOA 



number of chromosomes is not reduced in the maturation-divisions 

 although the amount of chromatin is reduced by half. 



It must be pointed out also that in many cases of conjugation 

 among Protozoa no maturation processes are known. Thus all of the 

 Flagellidia, with their complicated division of labor and high grade of 

 sex-differentiation, offer not a single instance of nuclear reduction, 

 and the conclusion is suggested that the maturation of forms in other 

 divisions of the Protozoa show no genetic relation to analogous pro- 

 cesses in Metazoa, but are independent expressions of the same un- 

 known vital forces which cause the formation of polar bodies, or the 

 double division of tetrads. 



C. GENERAL CONSIDERATIONS 



In the foregoing review of the phenomena of conjugation and 

 maturation, it is only too apparent that many gaps in the series, and 

 the incompleteness of the observations in various instances cited, 

 prevent any far-reaching generalizations. It may be pointed out, 

 however, that more or less similar .conditions characterize all of the 

 diverse phenomena, and afford a basis for future explanations. 



The various conjugation-phenomena seen in the Protozoa seem to 

 show that each cycle starts with a certain potential of vitality which 

 is gradually exhausted in the vegetative activities of the long line of 

 individuals formed by simple division, or by spore-formation. As 

 Biitschli long since suggested, such a cycle can be compared to the 

 ontogeny of a metazoon where the somatic cells, starting with an 

 initial vitality, finally die from senile degeneration. An important 

 difference, however, lies in the fact that each protozob'n has the 

 inherent power of conjugation, and when senile degeneration has 

 progressed to a certain extent, the vitality can be restored by this 

 process. The most careful observations on the Protozoa, as upon all 

 other animals and plants, have failed to demonstrate the nature of this 

 renewal of vitality, or the reason why the temporary or permanent 

 union of two exhausted cells should result in one or two rejuvenated 

 ones. 



An explanation of the many diverse modifications in the form of the 

 conjugants, and of the various maturation-phenomena, does not, how- 

 ever, appear at the present time quite so far out of reach, for these 

 phenomena can be observed, and the similar characteristics point 

 toward a common interpretation. The evidence in Protozoa, while 

 not conclusive, certainly points, I believe, as Minot long since 

 suggested, toward the phenomenon of decreasing vitality as the 

 underlying condition which indirectly brings about conjugation, sex- 

 differentiation, and maturation. 



