II] 



ASCOMYCETES 



45 



was proposed. The occurrence of a brachymeiotic reduction has since been 

 observed in several other fungi, and has also been in several cases denied. 



Chromosome Association. There are a number of fungi, of which 

 Phyllactinia Corylea is perhaps the most fully studied, in which no change in 

 the chromosome number takes place 

 throughout the life-history. In Phyl- 

 lactinia, Harper showed in 1905 that 

 the chromosomes remain visible in 

 strands attached to the central body 

 throughout the resting stages. In 

 each of the nuclei of the developing 

 ascus eight such strands can be 

 clearly seen (fig. 14), and their asso- 

 ciation in pairs can be followed, so 

 that they appear as eight strands in 

 the spireme stage, and as eight 

 chromosomes on the spindle. In the 

 smaller sexual nuclei Harper found 

 a similar arrangement. It would 

 thus appear that in this fungus chro- 

 mosome association follows directly 

 on nuclear fusion, so that the fusion 

 of two nuclei with n univalentstrands 

 produces not a nucleus with 2n 

 strands, but one with n bivalent strands. If this is so in the oogonium the 

 nuclei which fuse in the ascus will each possess n bivalent chromosomes, 

 and the definitive nucleus will show not 4^ univalerrt chromosomes but 

 which are quadrivalent. In the same way neither meiosis nor brachymeiosis 

 will affect the chromosome number, but will affect only the valency of the 

 chromosomes. This is the case in Phyllactinia, where in all stages of the 

 three divisions in the ascus, eight chromosomes are found. It must be 

 noted, however, that in cases where no fusion occurred in the life-history 

 except that which immediately precedes meiosis, the chromosome number 

 would similarly remain unchanged ; the latter interpretation has been 

 urged as evidence that forms in which the chromosome number is unaltered 

 throughout the life-history are therefore necessarily without a fusion in the 

 oogonium. 



The Theory of a Single Nuclear Fusion. The possibility that the 

 fusion in the ascus is the only nuclear fusion in the life-history of the 

 Ascomycetes, and represents the postponed union of sexual nuclei which 

 had become associated in fertilization was first mooted by Raciborski in 

 1895, in a letter to Professor Harper, and was published in 1896. This view 



Fig. 14. Phyllactinia Corylea (Pers.) Karst.; 

 fusion nuclei in ascus showing eight chromatin 

 strands attached at a common point; after 

 Harper. 



