84 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



ascus. These would then be in turn distributed in the triple division 

 which follows. The apparent number of chromosomes in each nucleus 

 throughout the life of the mildew would then be 8. The real number 

 in the nuclei in the ascogonium and ascogenous hyphse would be 16, 

 and in the primary nucleus of the ascus 32, as the size of these nuclei 

 indicates. 



It is a matter of great importance to determine, in the nuclear 

 divisions immediately following fertilization, whether, as would be 

 expected by analogy with the higher plants, the 16 chromosomes may 

 not be made out as distinct units. It seems probable, however, judging 

 from the nature of the process as seen in the fusion in the ascus, that 

 the union of nuclei with an organization such as is found in Phyllac- 

 tinia must always result in an immediate apparent reduction of the 

 chromosome number, though an actual reduction is only effected here, 

 as in the higher plants, by a process of synapsis, with its succeeding 

 reduction divisions. These latter considerations, of course, have only a 

 hypothetical value in the absence of an exact determination of the num- 

 ber of chromosomes in the cases involved, and it is important that the 

 number of chromosomes appearing in the first and the succeeding divi- 

 sions of the egg-nucleus should be determined. I have been unable so far 

 to find figures of this division which permit of counting the number of 

 chromosomes with certainty. The division figures in the young ascogo- 

 nium seem especially hard to find and are very liable to show the chro- 

 mosomes so closely bunched together that it is difficult to make out their 

 number. 



I have observed in a number of ascogonia, after fertilization in the 

 early stages of their growth, a small nucleus, one-third or less the diam- 

 eter of the ordinary nuclei, whose fate I have been unable to determine. 

 It is possible that the first division in the oogonium gives rise to a super- 

 numerary nucleus, such as Klebahn has described as being formed in 

 the germination of the zygospore of Closterium. It is, of course, also 

 of great importance to trace out in detail the behavior of the central 

 bodies here, both in the sexual fusion and the divisions which immedi- 

 ately follow, and I hope, either in the case of Phyllactinia or other 

 favorable material, to be able to throw further light on these questions. 

 At present we can conclude, from the number of chromosomes in the 

 division following synapsis in the ascus, that the somatic number is 8, 

 and, further, that at least 8 presumably bivalent chromosomes are pres- 

 ent in each of the nuclei which fuse in the ascus. 



If we attach the significance which I have indicated above to the 

 triple nuclear division in the ascus, and if we further assume that the 



