284 BIOLOGY OF THE PROTOZOA 



and black in appearance with complete loss of the usual vesicular 

 character (Fig. 145). This particular condition was relieved by 

 the use of electrolytes (K 2 HP0 4 , KC1, etc.) added to the usual 

 medium. In extreme old age in ciliates there is apparently a cessa- 

 tion of the intricate activities involved in cell division. Evidence of 

 this is the tendency to form monsters and the tendency of parts to 

 undergo degeneration, nuclei, motile organs, kinetic elements, etc., 

 in particular. 



Between the extremes of youth on the one hand and old age on 

 the other is a condition of cumulative differentiation termed sexual 

 maturity. In this condition phenomena occur which do not occur 

 earlier 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 (the author 

 has 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 division may account for 

 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 caudatum, or 

 posterior vacuole in P. aurelia (Unger, 1926). 



When such differentiation progresses to the point of isogamete 

 and anisogamete formation further constructive activities and repro- 

 duction are no longer possible, 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 pro- 

 ceeded far enough to seriously affect the general metabolism and 

 power of reproduction. This is demonstrated by experiments with 

 "split" pairs, or separation of two individuals recently united in 

 conjugation, an experiment first performed by Hertwig (1S89) and 

 later by Calkins (1904, 1919) and by Jennings (1909). Here an 

 individual, thus separated, continues with the same division-rate 

 that it would have had had it not conjugated. Yet the history of 

 isolation cultures with exceptions noted above shows that ultimately 

 if conjugation and parthenogenesis are continually prevented, the 

 race, like anisogametes, will die. 



