VITALITY 



507 



maturity. In this condition phenomena occur which do not occur 

 earher and the organization may become visibly altered. Thus 

 gregarines lose their attaching organs and become gamonts; the 

 physical condition of Paramecium changes to such an extent that 

 two individuals will fuse on contact at any part of the cortex (I 

 have observed an amorphous group of nine such partially fused 

 individuals) ; or the phenomena of plastogamy in general are possible 

 under such conditions of differentiation. 



With the protoplasm in this latter condition due to continued 

 metabolism further differentiations are possible and, carried out in 

 different directions, lead to specializations characteristic of gametes. 

 As Biitschli first suggested inequalities in di^■ision may account for 



Fig. 212. ^Paramecium caudatum in a period of depression and recovery bj' treat- 

 ment with salts. (After Calkins.) 



the differences in gametes, a possibility indicated by the more irri- 

 table anterior region of the ciliates, or by the more active pulsations 

 of the anterior contractile vacuole in Paramecium. 



When such differentiation progresses to the point of isogamete 

 and anisogamete further metabolic activities and reproduction are 

 stopped, and if fusion is prevented, the gametes die. With the 

 ciliates this is true only of the Vorticellidae. In other ciliates, 

 differentiations at sexual maturity have not proceeded far enough 

 to seriously affect the general metabolism and power of reproduction. 

 This is demonstrated by experiments with "split" pairs, or separa- 

 tion of two individuals recently united in conjugation, an experi- 

 ment first performed by Hertwig (1889) and later by Calkins 

 (1904, 1919) and by Jennings (1909). Here an individual thus 



