THALLOPHYTA 
325 
zoospore is an egg-shaped structure (Fig. 330). The anterior 
region is clear and is surrounded by a circle of cilia, while the 
remainder of the zodspore is colored green 
by the chloroplastid. The zodspore some- 
times contains a pigment spot. A zodspore 
is set free by the splitting of the wall of 
the cell in which it is produced. It swims 
about for a while, then attaches itself to 
some object by means of its anterior end, 
withdraws its cilia, produces a cell wall, and 

Fie. 331. Filament of 
Oedogonium 
Below isa large rounded 
odgonium, and above are 
four small cells, the an- 
theridia. (x 240) 
grows into a new 
filament (Fig. 330). 
Sexual reproduc- 
tion is the result of 
the fusion of an egg 
and a spermatozord. 
A spermatozoid is a 
small motile gamete 
which fuses with 
an egg. The eggs 
are borne singly in 
large, rounded cells 
known as oogonia 
(Fig. 331), while the 
spermatozoids are 
formed in special 
small cells called an- 
theridia (Fig. 331). 
An odgonium is a structure in which one 
or more eggs are produced, while an an- 
theridium is a structure in which sperma- 
tozoids are formed. 
The spermatozoids 
are similar to the zodspores except that 
they are smaller. When the egg is ready for fertilization, an 
opening appears in the wall of the odgonium, and the spermato- 
zoid enters through this and fuses with the egg cell (Fig. 833). 

Fia. 332. Plant of Oedo- 
gonium with two rounded 
o6gonia and five dwarf 
males attached to it 
Redrawn after Pringsheim 
