68 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



ascogenous cell. While in the growth of the vegetative hyphae, the 

 formation of sexual cells and the development of the ascogonium and 

 ascogenous hyphse nuclear division are followed by cell division, so that 

 uninucleated cells are formed, we have here a stage in which cell division 

 does not occur between the two nuclei of certain cells, and the two 

 nuclei remaining in the same cell mass fuse into one. Gerassimoff (26) 

 induced the interruption of cell division by suitably regulated inhibitive 

 stimuli (chilling, anesthetizing). The inhibition is self-induced in the 

 mildew by the operation of factors which are directed to the production 

 of a large cell with abundance of material for the formation of spores. 



But whatever the factor or factors may be which inhibit cell division 

 and thus leave the young ascus with two nuclei, this condition results 

 exactly as in Gerassimoff' s binucleated Spirogyra cells and Boveri's 

 sea-urchin eggs with the double number of chromosomes. The rela- 

 tive excess of nuclear material facilitates the rapid growth of the ascus 

 in size. The cells of Spirogyra do not complete the interrupted division 

 and thus reestablish the nucleo-cytoplasmic equilibrium, but grow larger, 

 not by pathological hypertrophy leading to death, but in a normal fashion 

 which permits of their indefinite further division and growth. The 

 ascus shows exactly the same physiological reaction to its increased 

 nuclear content. It immediately grows to a far greater mass than that 

 of any of the other cells of the ascocarp or mycelium. The evidence 

 is thus very strong that the doubling of the nuclear mass in the young 

 ascus is merely a preliminary to that growth of the ascus which is neces- 

 sary for its functioning as a spore-sac. The whole process is thus placed 

 in the category of nucleo-cytoplasmic regulations which are concerned 

 with maintaining an equilibrium between the factors of assimilation and 

 division in the cell. 



It is doubtless true that nucleo-cytoplasmic equilibrium is achieved 

 many times by an increased number of nuclei in the cell without their 

 fusion, especially in the algae and fungi. This is plainly the case in the 

 multinucleated perithecial cells of the mildews themselves and in the 

 multinucleated hyphae and reproductive cells of Pyronema and Asco- 

 bolus. In both of these latter cases increased size as well as increased 

 number of the nuclei especially characterize the large oogonia and the 

 ascogonia. In the multinucleated endosperm cells and in Nemec's (70) 

 binucleated root cells fusion may occur at once or the nuclei may remain 

 independent and divide again, whereupon nuclear fusion may occur. In 

 Gerassimoff 's (29) experiments fusion of the nuclei might or might not 

 occur. Hence it seems to be a matter of relative indifference, which 



