VORTICELLA 191 
well-being of the race. The process is, however, somewhat dif- 
ferent from conjugation 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 conjugation 
is then effected instead of a temporary one as in Paramecium. 
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. 
