244 FOUNDATIONS OF BIOLOGY 



fertilization, or SYNGAMY, occurs. Under usual conditions, 

 Paramecium, for instance, reproduces by fission two or three 

 times a day so that in a remarkably short period the one cell 

 is replaced by a host of descendants. Sooner or later, how- 

 ever, the individuals exhibit a tendency to unite temporarily 

 in pairs, or CONJUGATE. In this process complicated changes 

 take place in the nuclei of the cells, during which, after so- 

 called maturation phenomena, two pronuclei are established 

 in each individual of the pair of conjugants. Then one of the 

 pronuclei in each conjugant migrates over and fuses with the 

 stationary pronucleus of the other to form a synkaryon, or 

 fertilization nucleus, in each cell. After this the two Para- 

 mecia separate, reconstruct their characteristic vegetative 

 nuclear apparatus, and proceed to reproduce by division as 

 before. (Fig. 130.) 



This is fertilization in Paramecium, and it is generally con- 

 ceded that the primary significance of synkaryon formation 

 must be sought among unicellular forms of which this is an 

 example. Accordingly a large amount of experimental 

 breeding has been carried out on Paramecium and its allies. 

 The earlier results seemed to demonstrate conclusively that 

 Paramecium can divide only a limited number of times, say 

 a couple of hundred, after which the cells die from exhaustion 

 or SENILE DEGENERATION, unless conjugation takes place. 

 In other words, it was believed that periodic REJUVENATION 

 by fertilization is a necessity for the continuance of the life 

 of the race. And therefore, so the natural conclusion ran, 

 protoplasm is unable to grow indefinitely; there is an in- 

 herent tendency for the destructive phases of metabolism 

 to gain ascendency over the constructive, and fertilization 

 serves to maintain or restore the youthful condition and 

 thus secure the continuance of the race. 



In this connection, the life history of Paramecium from one 



