66 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



formation reunite to form a single nucleus with a doubled number of 

 chromosomes — 72 instead of 36. The result is identical with that in 

 Spirogyra. The larva produced from an egg so treated has abnormally 

 large cells. Boveri further observed that in all these variations the 

 superficial area of the nucleus and not its cubic content is proportional 

 to the number of chromosomes in it. 



On the ground of a general consideration of the relations of nuclear 

 and cytoplasmic masses, as well as his own, Gerassimoff's, and Boveri's 

 experimental results, Hertwig (45) extends these conclusions to all 

 cells as a general law of cell organization, which he calls the principle 

 of the nucleo-cytoplasmic relation. Nuclear masses and cytoplasmic 

 masses strive always to maintain a definite proportionality in which they 

 are in equilibrium. Any increase in the mass of either tends toward 

 producing a corresponding increase in the other; a reduction in one 

 necessitates a reduction in the other, in order that the nucleo-cyto- 

 plasmic equilibrium may be maintained. Hertwig considers this rela- 

 tion as one chiefly of mass, but it is plain that other factors are also 

 involved. A mere equilibrium of mass could be attained in the starving 

 Actinosphaeria by an equal and proportional reduction of each nucleus, 

 but instead of this the object is gained by the total destruction of certain 

 nuclei and the survival of others. In Spirogyra, also, the equilibrium 

 in the binucleated cells at least might be established by the later com- 

 pletion of the cell division which was artificially interrupted. It is 

 plain, then, that factors are present in the process which tend not only 

 to establish a definite nucleo-cytoplasmic relation of mass, but which 

 also determine the method by which this condition of equilibrium is 

 brought about. Equilibrium in the nucleo-cytoplasmic relation in the 

 case of enlarged cells may be brought about either by the presence of 

 two or more small nuclei or by the formation of a single nucleus of 

 proportionally greater size. 



If we now compare the cell-reactions experimentally discovered 

 by Hertwig and Gerassimoflf with those prevailing in the normal devel- 

 opment of the ascus, we shall find a striking similarity in all character- 

 istic features. We may consider first the ascus of the mildew. 



The ascus is to be developed as a relatively large cell to serve as 

 a storehouse, with an abundant supply of material for the formation 

 of ascospores ; and in order that the nucleo-cytoplasmic equilibrium may 

 be maintained, it must be provided with an excess of nuclear material 

 as compared with the other cells of the ascogenous hyphae and the asco- 

 gonium. There are several stages in this differentiation of the ascus 

 as to its nuclear content. It is binucleated from the first, while the 



