REPRODUCTION 161 



zoites, because of the preconj ligation fission {d~e). The micronuclear 

 divisions are similar to those that have been described for Para- 

 mecium caudatum and finally two pronuclei are formed in each con- 

 jugant. Exchange and fusion of pronuclei follow. In each excon jug- 

 ant, the synkaryon divides once to form the micronucleus and the 

 macronuclear anlage {k-l) w^hich develops into the "spireme ball" 

 and finally into the macronucleus (m-o). 



A sexual process which is somewhat intermediate between the 

 sexual fusion and conjugation, is noted in several instances. Ac- 

 cording to Maupas' classical work on Vorticella nehuUfera, the or- 

 dinary vegetative form divides twice, forming four small individuals, 

 which become detached from one another and swim about inde- 

 pendently. Presently each becomes attached to one side of a stalked 

 individual. In it, the micronucleus divides three times and produces 

 eight nuclei, of which seven degenerate; and the remaining nucleus 

 divides once more. In the stalked form the micronucleus divides 

 twice, forming four nuclei, of which three degenerate, and the other 

 dividing into two. During these changes the cytoplasm of the two 

 conjugants fuse completely. The wandering nucleus of the smaller 

 conjugant unites with the stationary nucleus of the larger conjugant, 

 the other two pronuclei degenerating. The synkaryon divides several 

 times to form a number of nuclei, from some of which macronuclei 

 are differentiated and exconjugant undergoes multiplication. 



Another example of this type has been observed in Metopus es 

 (Fig. 80). According to No land (1927), the conjugants fuse along the 

 anterior end (a), and the micronucleus in each individual divides in 

 the same way as was observed in Paramecium caudatum (h-e). But 

 the cytoplasm and both pronuclei of one conjugant pass into the 

 other (/), leaving the degenerating macronucleus and a small 

 amount of cytoplasm behind in the shrunken pellicle of the smaller 

 conjugant which then separates from the other (j). In the larger 

 exconjugant, two pronuclei fuse, and the other two degenerate and 

 disappear (g, h). The synkaryon divides into two nuclei, one of which 

 condenses into the micronucleus and the other grows into the macro- 

 nucleus (i, k-m). This is followed by the loss of cilia and encystment. 



Automixis. In certain Protozoa, the fusion occurs between two 

 nuclei which originate in a single nucleus of an individual. This 

 process has been called automixis by Hartmann, in contrast to the 

 amphimixis (Weismann) which is the complete fusion of two nuclei 

 originating in two individuals, as was discussed in the preceding 

 pages. If the two nuclei w^hich undergo a complete fusion are present 

 in a single cell, the process is called autogamy, but, if they are in two 



