NUCLEAR FUSION IN THE ASCUS. (i^ 



Other cells mentioned are uninucleated ; and, further, its two nuclei fuse 

 with the union of all their corresponding parts to form a single larger 

 nucleus, which in turn grows with the further growth of the ascus. 



The binucleated condition of the young ascus, we may conceive, 

 is due to an inhibition of cell division, due in turn, perhaps, to a cul- 

 mination in the process of extra feeding of the ascogenous cells, which 

 the whole structure and development of the ascocarp is calculated to 

 bring about. Cell division and nuclear division are quite independent 

 processes in the development of the ascogonium and ascogenous hyphae, 

 as we have seen above. For a time the ascogonium and the ascogenous 

 hyphae, in their rapid growth, are multi-nucleated, but in the end cell 

 division overtakes nuclear division and the whole system comes to con- 

 sist of uninucleated cells, except the cells which are to become asci. 

 The binucleated condition remains in them simply because cell division 

 is inhibited at just this stage of development. It seems probable that 

 these ascogenous cells are differentiated as such simply on the basis of 

 their more favorable position for nutrition, and that this excessive nutri- 

 tion is the stimulus which inhibits cell division. 



It is a frequently expressed conception that the stimulus to cell 

 division is given in a certain maximal size of the cell, which, when it is 

 attained by the growth of the daughter cells, results in certain tensions 

 which set in operation the mechanism of karyokinesis. Shaper (85) has 

 pointed out that increased volume results in a relative diminution of the 

 surface area as compared with the mass of the cell, and since all nutrition 

 comes through the surface, a stage will be reached when assimilation 

 and dissimilation will balance each other and growth will cease. Cell 

 division now occurs, and by the formation of two smaller daughter cells 

 a relation of volume and surface area favorable for growth will again 

 be established. Considering the growth of the animal egg, Lubosch 

 (58) points out that the special provisions for its nutrition, nurse cells, 

 etc., may have the effect of inhibiting cell division by furnishing such 

 a rich food supply that the relative diminution of absorbing surface will 

 be more than counterbalanced and the cell may continue to grow with- 

 out dividing as long as the excessive food supply is available. The case 

 of the ascus is similar, and it seems entirely reasonable to assume that 

 the excessive food supply prevents the separation of the two nuclei in 

 the young ascus by the formation of a cell wall. The relative excess 

 of nuclear material thus accumulated favors the further growth of the 

 cytoplasm independently of the rate of food supply, and thus we get 

 further rapid increase in size of the ascus cell. We have thus, in the 

 formation of the ascus, a definite change in the habit of growth of the 



