238 SEX IN MICROORGANISMS 



gated pairs of chromatids appear (19) and four chromatids go to 

 each pole (20). This is the third nuclear division, which corresponds 

 to the second in other ciliates; it seems to be the "reduction" division. 

 In the fourth and last division (21 to 24) only four chromosomes 

 appear, and these divide to send four to each pole of the spindle (24). 



The peculiar behavior of the chromatids in the first meiotic 

 division has caused much confusion about chromosome numbers in 

 meiosis. Turner considered the granules in the prophase of the first 

 meiotic division (15) to be chromomeres, and stated that they usually 

 are arranged in about eight groups. In the anaphase (16), the chroma- 

 tin is still in the form of chromomeres. In the next division four pairs 

 of chromatids appear (19). Presumably each pair is derived from 

 four chromomeres. If the chromatin bodies which appear during 

 the first maturation division are chromomeres, it is possible to fit the 

 whole series of divisions into a scheme comparable to that in the 

 Metazoa. 



Gregory (1923) had still more difficulty with Oxytricha fallax, 

 since she counted twenty-four dumbbells in the first maturation meta- 

 phase, twelve going to each pole; in the second division twenty-four 

 dumbbells again appeared and twelve went to each pole; in the third 

 division twelve dumbbells appeared and twelve granules separated to 

 the poles. This case seems to involve double reduction or an extra 

 division of chromatids between the first and second divisions. How- 

 ever, Kay (1946), in Oxytricha bifaria, reported forty-eight granules 

 in the first maturation division, twenty-four going to each pole, and 

 twenty-four chromosomes in the second, twelve going to each pole. 

 She considered that reduction took place in the second division. 



The tendency for protozoan chromosomes to appear as granules 

 in one stage of mitosis and as more solid strands at another is illus- 

 trated by conjugation stages in Nyctothenis cordijoriuis (Wichter- 

 man, 1937). Two of these stages are shown in Fig. AK (1, 2). In (1) 

 the anaphase in the conjugant at the left shows a group of granules 

 passing to each pole in the third pregametic division, which produces 

 the pronuclei. Metaphases of this division also show a similar number 

 of granules. In the right-hand conjugant of this figure, the telophase 

 of this division shows a "spireme." In the other figure (2) are seen 

 the fusing pronuclei where there seems to be a single thread-like 

 chromosome in each pronucleus. Noland (1927) found a parallel 

 situation in Metopus sigmoides. These figures illustrate some of the 

 difficulties involved in interpreting chromosome numbers in ciliates. 



