24 PROTOPLASM AND PLANT CELLS 
the cell into two parts. This membrane splits and be- 
tween these two plasma membranes is secreted the first 
layer of the cell wall (middle lamella). It is of interest 
to note that mitotic nuclear division is essentially the 
same in animals and plants. In the former, however, 
centrosomes are usually present while they are lacking in 
plants except in some of the lower groups. 
34. In internal cell formation the nucleus usually 
divides several times before the cytoplasm separates. 
Usually the new cells are formed almost simultaneously 
gm, in this case. In many cases the cleavage of 
) <ei8? the cytoplasm is such that all of it is used up 
; (ala) in forming the new cells, the spindle fibrillae 
taking no part in the process. In other cases, 
as in the formation of ascospores in the ascus, 
the kinoplasmic fibrillae radiating from the 
F . 8.— - . . . 
Internal cell Centrosome outline the new cell in the midst 
formation. of the mass of cytoplasm, leaving much of 
the latter outside of the new cells, the so-called epiplasm. 
35. Cell formation by union is in the main the opposite 
process to that by division. The union of the cytoplasm 
of the uniting cells is usually followed by the union of the 
nuclei to form one nucleus. If the cells are naked the 
process is comparatively simple, but when enclosed in 
walls the cells must either escape before uniting, or open- 
ings must be made in the walls so that one cell can pass 
into the other. By the union of the two nuclei the num- 
ber of chromosomes is doubled and remains at this so- 
called diploid number until by a peculiar modification of 
the mitotic process (the reduction division or meiosis) the 
number is reduced to the original (or haploid) number. 
Laboratory Studies. (a) Scrape off, after moistening with 
alcohol, a little of the young white moldy growth on a lilac 
leaf (powdery mildew) or of similar mildews on cherry shoots 
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