20 



PEOTOPLASM AND PLANT CELLS 



with modifications, Viz., fission, in which the cell divides 

 into two adjacent parts which may or may not remain at- 

 tached, and internal cell formation, in which the proto- 

 plasm within the cell divides into several cells which 

 eventually escape from the old cell wall as naked cells 

 (zoospores and motile gametes) or form new walls for 

 themselves within the old wall and become free on the 

 rupture or decay of the old wall. The latter type in- 

 cludes cases in which all the protoplasm is used up in 

 forming the new cells, as in zoospore formation, as well 

 as those in which only a part is so used, the remainder 





Fig. 7. — Karyokinesis (mitosis). 



lying between the new cells and the old wall, as in the 

 formation of ascospores within the ascus. Several forms 

 of fission may be distinguished. The commonest type 

 is that in which the protoplasm of the cell separates 

 into two parts that secrete a new wall between them, 

 the new cells thus remaining attached to each other. 

 The new separating wall may be formed as a ring-like 

 thickening on the old wall which gradually increases in 



