FERTILIZATION 



291 



on Humaria rutilans 1907, 1908; Carruthers on Helvella crispa 1911; 

 Blackman and Welsford on Polystigma rubrum 1912). By the above 

 investigators this early fusion was regarded as a sexual one, that in the 

 ascus being vegetative in nature; and some described a "double reduc- 

 tion" in the ascus to compensate for the two nuclear fusions. (See 

 p. 223.) 



In a series of somewhat later researches another group of observers 

 found the evidence for an early fusion to be very unsatisfactory, and' 

 concluded that the only nuclear union in the life cycle is that occurring 

 in the ascus: with Dangeard they saw in this union the sexual act. Fur- 

 thermore, no "double reduction" was found in the ascus. Among the 

 researches supporting this view, which now appears to be the more 

 probable, may be cited the following: Claussen on Pyronema confluens 



FIG. 116. 



A, nuclear fusion in the ascus of Peziza vesiculosa. (After Dangeard, 1894.) B, cell 

 fusion in seciospore sorus of Phragmidium speciosum. After Christman, 1905.) 



1907, 1912; Schikorra on Monascus 1909; W. H. Brown on Pyronema 

 confluens 1909, Lachnea scutellata 1911, and Leotia 1910; Faull on Lab- 

 oulbenia 1911, 1912; Blackman on Collema pulposum 1913; Nienburg on 

 Polystigma rubrum 1914; Ramlow on Ascophanus carneus and Ascobolus 

 immersus 1914; Brooks on Gnomonia erythrostoma 1910; McCubbin on 

 Helvella elastica 1910; H. B. Brown on Xylaria tentaculata 1913; and Fitz- 

 patrick on Rhizina undulata 1918a. 



As the two nuclei fuse in the young ascus Harper (1905) observed in 

 the case of Phyllactinia ccfrylea that not only the chromatin systems but 

 also the nucleoli and "central bodies" (centrosomes), upon which the 

 chromatin strands converge, unite. In the Ascomycetes generally the 

 fusion nucleus, or "primary ascus nucleus," undergoes three successive 

 mitoses to form the eight ascospore nuclei, the spore walls in each case 

 being formed in association with the curving astral rays which focus upon 

 the centrosome. (See p. 80.) 



