20 PROTOPLASM 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 
Fie. 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 
