282 



INTRODUCTION TO CYTOLOGY 



the third mitosis, such a double reduction being required by the two 

 nuclear fusions supposed to occur in the life cycle (see p. 230) (Fig. 163). 

 A compromise is suggested by Tandy, who reports that only some of the 

 sexual nuclei fuse in the ascogonium, so that the ascogenous hyphse 

 have both monoploid and diploid nuclei and give rise to asci in which the 

 definitive nucleus is therefore sometimes diploid and sometimes tetra- 

 ploid. In diploid asci, meiosis is accomplished in the first two mitoses, 

 whereas in tetraploid ones three mitoses are required to produce mono- 





6 7 8 9 10 



Fig. 164. — Meiosis in basidium of Cortinarius cinnamomeus. 1, two nuclei in young 

 basidium. 2, fusion nucleus. 3-5, prophases of division 7; four bivalents. 6, early 

 anaphase I. 7, telophase I. 8, telophase //; four chromosomes in each group. 9, four 

 spore nuclei still in basidium; sterigmata forming. 10, mitosis in one of the four spores. 

 {After Wakayama, 1930a.) 



ploid spore nuclei. An interesting light is thrown upon this problem by 

 the fact that segregation of Mendelian factors (hence presumably of 

 homologous chromosomes) in the asci of Neurospora occurs in either of the 

 first two mitoses but not in the third (B. O. Dodge, 1927 et seq.; Lindegren, 

 1932, 1933). 



In the basidiomycetes, meiosis follows immediately upon the fusion 

 of nuclei, which ordinarily occurs in the basidium or its homologue. 

 In hymenomycetes, the fusion nucleus subdivides twice to form the 

 nuclei of the four basidiospores (Fig. 164), which thus carry the reduced 

 chromosome number.^^ Cases are known in which no fusion occurs in the 

 basidium, two spore nuclei being formed without meiosis (Bauch). In 



" Sappin-Trouffy (1896), Juel (1898a), Holden and Harper (1903), Maire (19056), 

 Guilliermond (1910), Kniep (1911, 1913), Levine (1913), Lindfors (1924), Bauch 

 (1926, 1927), Wakayama (1930o) and others. 



