230 INTRODUCTION TO CYTOLOGY 



of a "dikaryon") in the ascogenous hyphse which grow out from the asco- 

 gonium. In each ascus the two associated nuclei finally fuse. Accord- 

 ing to this interpretation/ therefore, the nuclear fusion is merely delayed : 

 instead of occurring in the ascogonium, it takes place in the ends of 

 branches produced by the latter, namely, in the asci; and this fusion may 

 be regarded as sexual in nature, as originally stated by Dangeard (1894). 



Other observers have claimed that there are two nuclear fusions in 

 the ascomycete life cycle : in addition to the fusion easily observable in the 

 ascus, it is contended that an earlier fusion occurs in the ascogonium after 

 the entrance of the contents of the antheridium,^ or in the ascogonium 

 when the antheridium is functionless or absent,^ or in vegetative cells 

 when the ascogonium is functionless or absent. ^^ In the ascomycetes 

 generally the fusion nucleus, or ''primary ascus nucleus," initiates three 

 successive mitoses to form the eight ascospore nuclei. A reduction in 

 chromosome number occurs in the course of these mitoses. Adherents of 

 the view that there are two nuclear fusions in the life cycle claim that 

 there is a "double reduction" in the ascus (p. 281). In certain yeasts it 

 has been shown that the production of ascospores is preceded by a union 

 of two cells and a fusion of their nuclei, the fusion nucleus dividing to form 

 the spore nuclei (see Guilliermond, 1920n). 



In the hymenomycetes it has long been known that a fusion of two 

 nuclei occurs in the basidium, itself the terminal cell of a hypha with 

 binucleate cells, prior to the formation of the four basidiospore nuclei 

 (Fig. 164). The discovery of the origin of the binucleate condition is 

 more recent. ^^ 



In certain species of several genera, including Co'prinus, Aleurodiscus, 

 PanoBolus, and Schizophyllum, it has been shown that carpophores, 

 although they will sometimes develop on a mycelium from a single 

 spore, appear freely only when mycelia from different spores are inter- 

 mingled. Unions then occur between uninucleate hyphae of different 

 strains and initiate the binucleate dikaryophase. According to Buller, the 

 migrating nucleus divides after entering the adjacent hypha, one of the 

 daughter nuclei then moving on to the next cell to repeat the process. In 

 this way the originally uninucleate mycelium becomes progressively 

 "diplodized. " In the resulting binucleate mycelium the paired nuclei 



' P. Claussen (1907, 1912), W. H. Brown (1909, 1910a, 19116), Faull (1911, 1912), 

 Nienburg (1914), Ramlow (1914), F. Brooks (1910), McCiibbin (1910), H. B. Brown 

 (1913), Fitzpatrick (1918a), Frey (1924), S. G. Jones (1925), Delitsch (1926). 



* Harper (1895-1905), Blackman and Fraser (1905), Gwynne-Vaughan and 

 Williamson (1931, 1932). 



9 Blackman and Fraser (1906a), Fraser (1907), Welsford (1907), Dale (1909), 

 Gwynne-Vaughan and Williamson (1930). 



i» Fraser (1907, 1908), Carruthers (1911), Blackman and Welsford (1912). 



11 Kniep (1915 el seq.), Bensaude (1918), Lehfeldt (1922), Mounce (1921, 1922), 

 Hanna (1925, 1928), Sass (1929), Buller (1930, 1931). 



