624 FERTILIZATION 



into the apposed conjugant. This macronuclear exchange results in each 

 exconjugant possessing half of both macronuclei. Their eventual de- 

 composition makes the exchange difficult to explain on functional 

 grounds. Summers and Kidder suggest that it may represent a "remi- 

 niscence of a more primitive protozoan condition before the separation 

 of trophic nuclear materials from the germinal materials." 



The odd elongations of degenerating macronuclear chromatin into 

 ribbon or rod-like fragments in Paramecium may be an abortive attempt 

 at a similar process. 



Conjugant Meiosis 



Because the general course of conjugation, as outlined by Maupas 

 (1889) is followed by the vast majority of ciliates so far studied, it is 

 convenient to use this outline in reviewing the process. His eight stages 

 are as follows: 



Stage A, in which the micronucleus swells and prepares for division; 



Stage B, the first meiotic or maturation division; 



Stage C, the second meiotic division; 



Stage D, the third nuclear division, which produces the pronuclei; 



Stage E, that of mutual exchange and the union of pronuclei; 



Stage F, the first metagamic (amphinuclear) division; 



Stage G, the second metagamic division; 



Stage H, subsequent reorganization. 



Stages A, B, C, and D are concerned with preparation for syngamy. 

 This preparation includes meiosis and the formation of pronuclei (see 

 Fig. 147). Stage E is the climax of the entire process, wherein the act 

 of fertilization is consummated. Stages F, G, and H are concerned with 

 the reorganization of the body and the reestablishment of the usual vege- 

 tative form. 



Among ciliates that normally possess more than one micronucleus 

 there is little uniformity in the number of nuclei that undergo the two 

 meiotic divisions. In many forms all micronuclei enter the first meiotic 

 division. Then all products of this division may divide again, or various 

 numbers of them may be resorbed (see Calkins, 1933, p. 295). In 

 Dileptus gigas, however, Visscher (1927) has shown that only one of 

 the large number of micronuclei undergoes maturation. 



Two to eighteen micronuclei have been described as entering the first 



