ALTERNATION OF GENERATIONS. 8 1 



division of nuclei can, however, at the most be regarded as no more 

 than a mere beginning 1 as compared with the indefinite series of conju- 

 gate divisions occurring in the rusts. 



As compared with the rusts, then, we have in the mildews and 

 Pyronema no series of binucleated cells formed by conjugate division, 

 and we do find positive evidence that the sexual nuclei fuse in the 

 oogonium. Still, I am convinced that in the Ascomycetes as well as 

 in the rusts we have a true alternation of generations. The explana- 

 tion of these differences, combined with so much of general similarity 

 between Ascomycetes and Basidiomycetes, is given in a further point 

 of difference between the two groups. 



In the teleutospore of the rusts and in the basidium, as Blackman 

 believes, we have a synapsis stage followed by a double division of the 

 nucleus leading to the formation of four spores. The parallelism 

 between the teleutospore and spore mother cells of the higher plants 

 is thus complete, and it is justifiable to assume that the first and second 

 divisions in the promycelia and basidia are respectively heterotypic and 

 homceotypic divisions. On the other hand, in the ascus following the 

 very conspicuous synapsis described above, we have a triple division 

 of the primary nucleus of the ascus leading to the formation of eight 

 ascospore nuclei and typically to the formation of eight spores. The 

 synapsis stage suggests that chromosome reduction occurs in the ascus ; 

 but in view of the absolutely universal occurrence of only two divisions 

 associated with chromosome reduction elsewhere in both plant and 

 animal kingdoms, this triple division in the ascus must be regarded as 

 a most aberrant occurrence, and has led to very great hesitancy on my 

 part in my earlier investigations in assuming the possibility of an alter- 

 nation of generations in the Ascomycetes comparable to that in the 

 higher plants. With the discovery in Phyllactinia that the chromo- 

 somes are not only permanent structures of the cell, but that they each 

 have permanent and distinct connection with the central body through 

 the resting condition of the nuclei as well as through the processes of 

 both nuclear fusion and division, combined with the evidence which has 

 been accumulating so rapidly from all sources that, even when appar- 

 ently not connected with the centers, the chromosomes still maintain 

 their identity in all nuclei, both of animals and plants, it becomes evident 

 that the triple division, in connection with chromosome reduction in the 

 ascus, as compared with a double division everywhere else among plants 

 and animals, becomes a fact of still more fundamental importance. 

 There is general agreement at present that the chromosomes of the 

 spore mother cells are bivalent structures and that both heterotypic and 



