626 FERTILIZATION 



maturation division of different ciliates. Variation in the number of 

 nuclei involved in the maturation divisions occurs within the same species, 

 as well as among different species. As only two pronuclei function in 

 fertilization, and as these two are known to be sister nuclei in many 

 ciliates, only one micronucleus really needs to undergo maturation. 



STAGES A AND B, THE FIRST MEIOTIC DIVISION 



In the earlier accounts of conjugation, the first maturation division 

 in several ciliates was said to be not greatly different from ordinary 

 vegetative mitosis. Recent accounts, based on careful cytological studies, 

 show marked peculiarities in the prophase of the first maturation division. 

 It seems possible, therefore, that more detailed studies, with improved 

 techniques, may reveal these distinguishing prophase stages in all ciliates. 

 The fact that such a careful worker as Maupas failed to observe the 

 highly characteristic changes that occur in Euplotes patella (Turner, 

 1930) lends weight to this possibility. 



In the vast majority of ciliates, the prophase of the first maturation 

 division is highly characteristic and presages the coming reduction. In 

 some ciliates the micronucleus takes on the form of a crescent, or comma, 

 during the prophase, and this appearance is sufficiently characteristic to 

 be recognized as a general type. Among ciliates that exhibit the crescent 

 formation are various species of 'Paraniechau and the Vorticellidae. 



There is little agreement as to the number of chromosomes in Para- 

 mecium or even as to the method by which the crescent is transformed 

 into the metaphase spindle. The chromosome number in all species is 

 surely larger than the 8 or 9 given by Hertwig (1889) for P. aurelia, 

 and probably less than 150, which has been attributed to P. caudatum. 

 Calkins and Cull (1907) suggested that the 165 or more small chroma- 

 tin rods or fibers seen in P. caudatum are comparable to the physical 

 counterpart of the individual genes of higher animals. Aggregates of 

 these would represent a chromosome in cases where chromosomes are 

 formed. Perhaps the 32 chromomeres of Euplotes patella (Turner, 

 1930) also represent 32 genes, although one would expect this highly 

 specialized hypotrich to have more genes than the more primitive holo- 

 trich. 



Whatever the nature of the chromatin elements in Paramecium may 

 be, these investigators, as well as Dehorne (1920), show that the first 



