VORTICELLA 171 



the rejuvenating act of conjugation. It goes on, however, some- 

 what differently than in Paramecium. An individual divides 

 into from two to eight parts. These free themselves from the 

 stalk, acquire each a basal band of cilia, and swim about in 

 the water until they come in contact with individuals of the 

 ordinary kind, with which they fuse. A permanent conjuga- 

 tion is then effected which is followed by renewed activity in 

 reproduction. 



Conjugation, it will be noticed, while it is not a sexual 

 process, is closely allied to such a process, and it is probably 

 through it that sexuality arose in the organic world. In Para- 

 mecium and Vorticella we have two important steps in the 

 development of sexuality. In the former animal the conjugat- 

 ing individuals are of the same size, or isogamous, and the 

 fusion of the two individuals is temporary, while in the latter 

 they are of different sizes, or heterogamous, and the fusion is 

 permanent. As a result of this differentiation in Vorticella 

 one of the conjugating individuals is a large, passive form, 

 while the other is a small, active, motile form, which finds and 

 fuses with the passive form. A distinct foreshadowing of the 

 two sexes which characterize the Metazoa is thus present. 



Exercise 3. Look among a large number of Vorticellas for con- 

 jugating and for dividing individuals. Observe them care- 

 fully and draw outlines of those observed. 



