NUCLEAR FUSION IN THE ASCUS. 63 



wall without leaving- a trace to indicate its path. The fertilization is 

 accomplished by the nucleus alone. 



Christman's (15) further discoveries in the same line and on a 

 related species form a most valuable extension of Blackman's results. 

 In Phragmidium speciosum Fr. Christman finds an actual and typical cell 

 fusion at the base of each row of secidiospores. The fusion is between 

 vertical hyphal cells whose bases diverge below, indicating that they 

 arise from quite widely separated hyphal branches. The fusing cells 

 are equal in size, and it is as an outgrowth of their combined apices that 

 the row of secidiospores takes its origin. Here, again, the nuclei of the 

 gametes do not fuse, but divide simultaneously to form the pairs of 

 nuclei found in the secidiospores. 



I shall have occasion to return to Blackman's and Christman's 

 results in other connections. Here we are chiefly concerned with the 

 fact that the time and degree of the visible combination of the sexual 

 prochromosomes is a variable matter. If the prochromosomes can 

 remain either in one nucleus with double chromosome number or in two 

 distinct nuclei through part or all of the sporophyte generation, it is 

 also possible that they may combine in one nucleus into bivalent chro- 

 mosomes and maintain their identity in this condition through the spo- 

 rophyte generation till a true reduction occurs in spore formation. It 

 is certain that with the nuclear organization described above the indi- 

 vidual chromosomes must be permanent structures, and that for every 

 chromosome unit which enters a given nucleus a corresponding chro- 

 mosome unit must reappear in the division of that nucleus. 



I have already (37, pp. 677-678) advanced the view that the forma- 

 tion of the primary nucleus of the ascus and the succeeding divisions 

 may correspond morphologically to the process of spore formation at 

 the end of the sporophyte generation in the ordinary cases of alternation 

 of generations. With the evidence presented above, that a reduction 

 of the number of chromosomes occurs in the formation of the primary 

 nucleus of the ascus, the evidence that the ascus, like the spore mother 

 cell of the moss or fern, represents the close of a sporophyte generation 

 is apparently strengthened; still, it is plain that, since the nuclei that 

 fuse in the ascus are themselves products of a nuclear fusion in the 

 oogonium which must double the chromosome number, we must look 

 further for a complete explanation of the processes here involved. 



J shall present further evidence on this point later ; but whether we 

 accept or reject the evidence for the existence of an alternation of gen- 

 erations in the Ascomycetes and their probable congeners, the Florideae, 

 the problem as to the nature of nuclear fusion in the ascus still remains. 



