6 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



centered on the polar body in the polar field of the cytoplasm. He 

 described the nucleus as showing thus a polar and an antipolar region 

 and its whole structure, including both chromatic and achromatic ele- 

 ments, as permanently centered on a definite pole. Rabl further con- 

 ceives the achromatic fibers of the resting condition as forming the 

 spindle figure in karyokinesis, and thus the basis is given for a mechan- 

 ical conception of division on the assumption of the contractility of the 

 fibrillar elements of the nucleus and cytoplasm. 



Rabl's conception of the polar organization of the nucleus and of 

 the mechanics of nuclear division was adopted by Flemming and is 

 given its fullest expression in his discussion of the mechanics of cell 

 division in the tissue cells of the salamander (25, pp. 715-749). 



Boveri (10), on the ground of his observations on the eggs of 

 Ascaris, opposed the doctrine of a persistent connection between the 

 chromatic elements and the centrosome, and maintained that a new con- 

 nection between chromosomes and poles is established in the prophases 

 of each succeeding cell division. With the development of the doctrine 

 of the individuality of the chromosomes this question has gained still 

 more fundamental importance, and in this connection I shall discuss, 

 further on, the observations of some later investigators. 



My earlier work (38) on the dividing nuclei in the ascus led me 

 to suspect that a permanent connection of chromatin elements and cen- 

 tral body exists in the mildews, and I have described and figured such 

 a condition in certain resting stages of the nuclei. In extending my 

 studies to the genus Phyllactinia I have found a more favorable form 

 with somewhat larger nuclei and have been able to trace a continuous 

 connection of centers and chromatin in both resting, fusing, and dividing 

 nuclei. 



Since various authors have expressed themselves as still denying 

 or as doubtful as to the existence of differentiated sexual cells in the 

 Ascomycetes and as to the consequent morphological relations of the 

 ascocarp, I shall precede my special description of the nuclear structures 

 and processes with an account of the development of the sexual organs 

 and the perithecium. I am convinced that on these much-vexed ques- 

 tions of the alternation of generations, relations of Ascomycetes, Basidi- 

 omycetes, red algae, etc., we shall only arrive at certainty by the most 

 painstaking study of the nuclear structures and processes and especially 

 the chromosome number at all stages of development, and I have 

 returned to the mildews for a further contribution to this subject because 

 in Phyllactinia I have found the most favorable ascomycete for the 

 study of the nuclei which I have yet encountered. PhyDactinia is 



