210 SEX IN MICROORGANISMS 



sequences as previously followed. One may find about as many varia- 

 tions within the orders Holotricha and Spirotricha as there are be- 

 tween them. 



In the process of conjugation, two individuals become attached 

 to each other and exchange gamete nuclei, after which they separate 

 and a new nuclear apparatus arises from the products of the fusion 

 nucleus, or syncaryon, in each exconjugant, while the old macronu- 

 leus gradually disappears. 



Conjugation in Farainecimn caudatum 



Conjugation as it occurs in Faramecium caudatum is usually con- 

 sidered typical. Accounts have been published by a number of 

 authors, for example by Gruber (1887) (under the name F. aurelia), 

 Maupas (1889), Calkins and Cull (1907), Dehorne (1920), Miiller 

 (1932), Penn (1937), Diller (1940b, 1950a) and Wichterman 

 (1953). The principal steps are shown diagrammatically in Fig. Z, 

 which is based on the accounts of Maupas and later authors. 



Two individuals become attached to each other along their oral 

 surfaces ( 1 ) ; then the micronucleus in each, after a long period of 

 preparation involving the "crescent" stage (not illustrated), undergoes 

 the first pregametic (meiotic) division (2). The two nuclei thus 

 formed quickly go through the second pregametic (meiotic) division 

 to produce four haploid nuclei (3). Three of these nuclei begin to 

 degenerate while the fourth divides to form the pronuclei (4). The 

 pronuclei usually take a position near the oral region, where a cone- 

 shaped protrusion, the paroral cone (Diller, 1936), extends from 

 each conjugant toward the other (4, 5). Through these cones the 

 migratory pronucleus (male) of each conjugant passes into the mate 

 (5), where it fuses with the stationary pronucleus (female) to form 

 the fusion nucleus or syncaryon (6). The conjugants then separate 

 (7), and the further changes take place in the exconjugants. In each 

 exconjugant the macronucleus gradually becomes transformed into 

 a skein (7 to 9) which breaks up into small fragments (10, 11). Mean- 

 while, by three successive mitoses the syncaryon gives rise to eight 

 small nuclei (7 to 9). Four of these begin to grow and are known 

 as macronuclear anlagen. As growth of the anlagen proceeds, three 

 of the other nuclei grow slightly, then begin to degenerate and even- 

 tually disappear, while the eighth nucleus becomes the functional mi- 



