242 SEX IN MICROORGANISMS 



together. In certain ciliates, however, conjugation is accompanied by 

 a regular exchange of portions of the macronucleus between the 

 conjugants, as described, for example, for Anoplophrya branchianim 

 (Collin, 1909). As shown in Fig. AL, each macronucleus becomes 

 much elongated and then projects itself into the cytoplasm of its 

 mate (5 to 8). The result is that each exconjugant contains half of 

 its own macronucleus and half of that of its partner (9). Summers 

 and Kidder (1936) found a similar condition in Anoplophrya or- 

 chestiae, and Macdougall (1936) in Chilodojiella labiata. Just why 

 certain ciliates should regularly exchange portions of their macro- 

 nuclei during conjugation would be difficult to explain. 



Differentiation of Gametes 



Often there is no obvious difference in the appearance of the 

 two pronuclei which form in a conjugant, but in many cases there 

 are more or less pronounced differences. Sometimes there is merely 

 a slight difference in size, as in Faraiuecmm caiidatiim (Maupas, 1889; 

 Calkins and Cull, 1907, and others), or there may be a special cyto- 

 plasmic area in connection with the migratory pronuclei as described 

 for Uropleptiis mobilis (Calkins, 1919). But perhaps the most ex- 

 treme case of gamete differentiation is that in Cycloposth'mm bipal- 

 viatinn (Dogiel, 1925) where the migratory pronucleus becomes 

 sperm-like with a long tail, breaking out of the endosarc into the 

 joined peristomal cavities and passing down the cv^topharynx to the 

 endosarc of its partner (Fig. AH, 4) . The differentiation of the male 

 gamete in this case is comparable to that in many Metazoa. 



Different Patterns of Behavior in Conjugation 



Some of the variations in the behavior of the nuclei during con- 

 jugation have been referred to in the previous pages. No attempt has 

 been made in this review to classify them. Dogiel (1925), in his 

 extensive review of conjugation, recognized five different types of 

 nuclear behavior leading to synkaryon formation and ten types of 

 nuclear reconstruction in exconjugants. Since there is often a great 

 deal of variation within any one species, as shown, for example, by 

 Diller (1948) for farmnechim trichhivi, any attempt to make use of 

 or expand Dogiel's classifications does not seem profitable. 



