82 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



homceotypic divisions are necessary to reduce them to the condition 

 found in ordinary somatic cells. The three divisions of the nucleus 

 of the ascus following each other in rapid succession, just as do the 

 heterotypic and homoeotypic divisions, suggest at once that the organ- 

 ization of the chromosomes in this case must differ from that in the 

 ordinary spore mother cell. The most natural assumption would seem 

 to be that the chromosomes are quadrivalent in the nucleus of the ascus 

 rather than bivalent, as in ordinary spore mother cells, and that in two 

 of the divisions in the ascus chromosomes are separated instead of in 

 one division, as in the ordinary case. Direct evidence as to whether 

 the reductions occur in the first and second or in the second and third 

 divisions is very difficult to obtain. I have not as yet been able to recog- 

 nize in the figures in the ascus the ordinary distinctions between hetero- 

 typic and homoeotypic divisions. 



In any case, however, the triple division certainly suggests that 

 two reductions may be necessary to bring the chromosomes of each 

 nucleus back to the ordinary number, and this implies that each of the 

 eight chromosomes, as they emerge from synapsis, represents four 

 somatic chromosomes. 



If from this standpoint we seek an explanation of such a pecul- 

 iarity in the chromosomes of the primary nucleus of the ascus, we are 

 led at once to the suggestion that they must have arisen by a double 

 nuclear fusion, such as actually occurs in the oogonium and in the ascus, 

 as I have described. We see thus that the assumption that the ascus, 

 with its triple division, is a spore mother cell, representing the stage at 

 which chromosome reduction occurs at the close of a sporophytic gen- 

 eration, leads naturally to the expectation of two nuclear fusions in the 

 development of the ascocarp. The universality with which the triple 

 division occurs in all asci so far studied is sufficient proof of its funda- 

 mental and primitive nature and that it occurs in a spore mother cell 

 following an apparent numerical reduction of the chromosomes, and a 

 synapsis stage is in perfect harmony with the assumption that it is neces- 

 sary for the true reduction of the chromosome number. 



The triple division of the primary nucleus of the ascus is a univer- 

 sally present characteristic of all the higher Ascomycetes. It occurs, 

 none the less, whether eight or a smaller number of ascospores are to 

 be formed, and that it is a process of fundamental importance is thus 

 most strikingly shown, since it necessitates, as we have seen, the destruc- 

 tion by disintegration of six of the eight nuclei formed in such cases as 

 Phyllactinia, in which but two spores are produced in each ascus. It 

 is evident in such cases that the three divisions form together a single 



