ALTERNATION OF GENERATIONS. 83 



process, and that they all are necessary for its accomplishment. If, for 

 example, the two first divisions alone were necessary to accomplish the 

 reduction of the chromosomes, we must suppose that the third division 

 would readily disappear when only four or fewer spores were to be 

 formed. The fact that all three divisions persist shows their necessity 

 for the process of reduction. 



We are confronted here with cases parallel to those in the higher 

 plants, where fewer than four macrospores are to grow and become 

 functional. In such cases we find a strong tendency to the persist- 

 ence of double division as such, the supernumerary macrospores 

 becoming abortive and being absorbed. The persistence of the triple 

 division in the ascus thus suggests in itself that the primary nucleus 

 of the ascus contains proportionally more chromosomes than the ordi- 

 nary spore mother cell in other sporophytes, and we are thus led to 

 expect, what we actually find, the two nuclear fusions in the develop- 

 ment of the ascocarp. 



Further, on analogy with the spore mother cells in the higher plants, 

 we must conclude that the apparent number of chromosomes appearing 

 in the divisions immediately succeeding synapsis represents the normal 

 somatic number, which in the case of Phyllactinia would thus be eight. 

 I have above pointed out the difficulties involved in counting the number 

 of chromosomes in the ordinary vegetative divisions in the mildews. 

 The best one can say at present is that the appearances are not against 

 the assumption that there are eight chromosomes on each half of the 

 spindle in such a figure as is shown in fig. 23. 



It is quite certain that there are eight or more chromatin strands 

 representing chromosomes in the nuclei which fuse in the ascus. The 

 process of nuclear fusion in the case of nuclei whose chromosomes are 

 permanently attached to the centers, as I have found them in the mil- 

 dews, leads naturally to their approximation side by side in pairs, and 

 it seems probable that the fusion of the male and female nuclei in the 

 oogonium, in which the same attachment between centers and chromo- 

 somes exists, would have the same result. The male and female chro- 

 mosomes would thus be brought together in pairs. If, as in the case 

 of other sexual fusions, the chromosomes brought together in fertiliza- 

 tion still maintain their identity till the stage of reduction at the close 

 of the sporophyte generation, we must conclude that the eight chro- 

 matin strands found in the nuclei of the ascus just before their fusion 

 are really double, and that in the fusion in the ascus these double chro- 

 mosomes, becoming approximated in pairs and passing through synap- 

 sis, become quadrivalent chromosomes in the primary nucleus of the 



