LIFE HISTORIES OF ALG^ 251 
becomes organized into from one to eight motile spores 
(zdospores, or swarm-spores) , each with four cilia and a 
red "eye-spot." The zoospore escapes by swimming 
through a small opening in the cell-wall, attaches itself 
by its ciliate end, and by a series of cell-divisions produces a 
new filament like the one from which it came. In this 
case it is evident that the zoospore, reproducing without 
cell-fusion, is an asexual spore, and that the mother-cell 
from which it came functioned as a sporangium. 
240. Sexual Reproduction. Other cells of the same 
filament that produced the asexual zoospores, may, by 
successive nuclear and cell-divisions, become divided into 
as many as 1 6 to 64, or even more, independent motile cells, 
each with a red "eye-spot," but with only two cilia. 
Like the zoospores, they escape by swimming through an 
opening in the wall of the mother-cell. Occasionally one 
of them comes to rest and begins germination,* but the 
process never continues far enough to produce a new plant. 
More commonly, two of these cells come together and fuse, 
showing that they are, in reality, gametes. Since the 
gametes are similar in size they are termed equal gametes, 
or isogametes. The fusion of two isogametes is called 
conjugation, to distinguish it from the fusion of un- 
equal gametes; it is essentially the same as fertilization. 
Whether or not reduction occurs, in the nuclear and cell- 
divisions that result in the formation of the isogametes, 
is not known. 
241. Germination. After the fusion of the gametes the 
zygote at once begins to increase in size, but soon its cell- 
wall becomes thickened, and then the protoplast divides 
into a number of swarm-spores, each of which, when set 
free, may develop into a new plant. 
