NUCLEAR FUSION IN THE ASCUS. 75 



condition found now in the rusts. Briefly, the hypothesis involves that 

 the development of conjugate nuclear division and the maintenance of 

 separate lines of nuclear descent in the ascogenous hyphse might tend 

 to give to the nuclear fusions in the ascus a sexual value in addition to 

 their original and more fundamental significance in maintaining the 

 nucleo-cytoplasmic relation in the enlarged ascus cell, and that thus, in 

 turn, parthenogenesis, and later even apogamy, may have resulted in 

 forms in which conjugate nuclear division in the ascogenous hyphse 

 had become established. Such an hypothesis as to the possible dis- 

 appearance of the fusion of sexual gametes and its replacement by 

 the independent fusion in the ascus of nuclei of separated ancestry 

 carries with it no implication that the ascus has become the morpho- 

 logical equivalent of an oogonium. The fusion in the ascus would be 

 only analogous to and not homologous with a true sexual fusion, nor 

 would this hypothesis affect in any way our conception of the mor- 

 phology of the ascocarp. The ascus is a new structure which originated 

 as an outgrowth of an ascogonium, which in turn was produced by the 

 germination of a fertilized egg. The two fusions must be expected to 

 coexist for a time in the same life cycle, as is actually the case in all 

 Ascomycetes whose nuclear history is fully known. 



As I have pointed out before, it still remains to determine how 

 many such cases there are among the Ascomycetes and whether the 

 genera mentioned are really apogamous or parthenogenetic or, as 

 Blackman's discovery suggests, whether they may not possess a fertiliza- 

 tion by the migration of vegetative nuclei in their initial cells. It is 

 certainly most highly desirable that we should have a full account of 

 the development of the ascogenous hyphae and the behavior of the 

 nuclei in some such type. In the case of the forms referred to above 

 also, in which Maire (59, 60) has reported that the asci arise from a 

 series of binucleated cells, this series in one case taking its origin in a 

 recurved hyphal tip such as usually gives rise directly to an ascus, H 

 is of the highest importance to know how the ascocarps originate. 

 Until this is settled it is difficult to judge with any certainty of the 

 significance of Maire's observations. It is to be hoped that in the near 

 future we may have a full account of the nuclear phenomena in the 

 development of the ascocarp of some parthenogenetic or apogamous 

 form. Unfortunately, the forms which have so far been reported as 

 developing their ascocarps in some other way than from a sexual appa- 

 ratus have not shown themselves favorable for the investigation c 

 nuclear phenomena. 



