FERTILIZATION 209 



" epidemics" of conjugation which may often be observed in 

 Protozoan cultures. In such cases conjugation may often be 

 artificially induced by regulating the character and amount of 

 the food supply (Jennings). 



It may be concluded, therefore, that among the Protozoa the 

 processes of reproduction and fertilization are not funda- 

 mentally related, and the primary significance of fertilization 

 must be sought in some other relation. This view is widely 

 accepted to-day and it consequently becomes necessary to 

 explain the practically universal association of the two proc- 

 esses among the Metazoa, the only exceptions being, as we 

 have seen above, the secondary and obviously derived instances 

 of normal parthenogenesis. The commonly suggested explana- 

 tion is the following. 



Whatever the real significance of fertilization may be, it 

 seems, for reasons which will appear later, a condition for 

 continued existence of specific forms of protoplasm that 

 occasionally some disturbance of its inner structure should 

 occur, such as would result from the mingling of the substances 

 of two distinct individuals. Among the single-celled organisms 

 this may occur at any time, whenever that action would form a 

 natural response to internal conditions of the organisms. 

 Among the Metazoa, on the other hand, such a complete fusion 

 of cells, and particularly of nuclei, can occur only when the 

 organisms are in the form of single cells, i.e., gametes, and 

 differentiation of the organism is at a minimum. Thus whereas 

 the two processes are originally distinct and unrelated in their 

 origin in the Metazoa, they have come to be related, and now 

 fertilization appears as the first step in reproduction. 



Another general hypothesis regarding the function and signifi- 

 cance of fertilization is the rejuvenation hypothesis, associated 

 chiefly with the names of Biitschli, Maupas, and Richard Hert- 

 wig. This is based to a large extent upon the phenomena of 

 the Protozoan life cycle. It involves as a starting point the 

 assumption, partly based upon observation, that protoplasmic 

 activity tends gradually to diminish in intensity, and that 

 associated with this diminution are certain morphological altera- 



