NUCLEAR FUSION IN THE ASCUS. 7^ 



the excessive assimilative activity in the ascus has inhibited nuclear 

 division just as in its earlier stages excess of nutrition inhibited cell 

 division to the extent of leaving the young asci with two nuclei. 



In the process of spore formation we have again a most striking 

 example of the controlling influence of the so-called nucleo-cytoplasmic 

 relation. The nucleus of the ascus divides to form two daughter nuclei, 

 and these in turn divide successively to form eight nuclei; but in 

 thus passing from the uninucleated to the multinucleated condition the 

 nucleo-cytoplasmic equilibrium is maintained. The two daughter nuclei 

 are proportionally smaller than the mother nucleus, and the four and 

 eight nuclei in the end bear approximately the same relation to their 

 cytoplasmic masses as did the primary nucleus of the ascus to the cyto- 

 plasm of the entire ascus. The two nuclei which become the centers 

 for the formation of spores grow to a somewhat larger size than the 

 remaining six, and accordingly the mass of cytoplasm included in the 

 two spores is more than one-fourth of that in the entire ascus. 



A careful study of the processes involved in the development of the 

 ascus leads thus to the conclusion that it is in its nature as a spore- 

 producing organ that we find the explanation of the various nuclear and 

 growth phenomena which characterize it. By inhibition of cell division 

 at a certain stage in the development of the ascogenous hyphse the ascus 

 is formed as a binucleated cell, and the excess of its nuclear content 

 makes possible a proportionate development of its cytoplasm. Its nuclei 

 grow and fuse and nuclear division is further inhibited, and thus the 

 relatively enormous size of the uninucleated ascus cell is atained. With 

 the diminution of food supply nuclear and cell division are resumed and 

 the uninucleated ascospores are formed, in which, again, the nucleo- 

 C3rtoplasmic relation is also maintained. 



A comparison of the processes thus described and analyzed with 

 those associated with fertilization elsewhere makes it still clearer that 

 the development of the ascus can not in any sense be compared with 

 that of the egg. Much attention has been devoted to the problem of 

 the conditions which lead to the formation of the immense yolk masses 

 of some animal eggs and the relation of yolk formation to the size of 

 the germinal vesicle. There is no question that we have here an illus- 

 tration of the principle of the nucleo-cytoplasmic relation, but in every 

 case the growth of the yolk mass is a purely vegetative process and is 

 the preparation for fertilization. In the ascus nuclear fusion is followed 

 by inordinate growth in the mass of the cell. ( Compare figs. 37 to 39, 

 which are magnified 2250, with figs. 53 and 54, which are magnified 

 1500.) With the possible exception of a few of the algae, in which 



