THALLOPHYTA 
become mature motile indi- 
viduals. In most cases the 
gametes of Chlamydomonas 
are all alike (isogametes), 
while in some cases they dif- 
fer in size (heterogametes). 
The similarity of the gam- 
etes and zoospores of Chlamy- 
domonas indicates that in such 
cases gametes have been de- 
rived from zoospores and 
that sexuality had its origin 
in the transformation of non- 
sexual zoospores into sexual 
gametes. 
Relationship of Chlamydo- 
monas. ‘The eyespot, the 
contractile vacuoles, and the 
flagella of Chlamydomonas are 
distinctive structures which 
are also presented by the 
flagellates. The fact that 
these structures are found in 
both Flagellata and Chlamy- 
domonas indicates that the 
groups are very closely re- 
lated. Chlamydomonas is 
more advanced than the Flag- 
ellata in that it has a sexual 
method of reproduction. 
ULOTHRIX 
Vegetative structure. Ulo- 
thrix is a green alga which 
is frequently abundant as a 
hairy growth on stones in 
321 

Fre. 324. Pleodorina illinoisensis 
This is one of the simpler members of the 
family V olvocaceae, in which the individual 
cells are arranged in colonies in the form 
of a hollow sphere. Vegetative reproduc- 
tion is by the division of a single cell to 
form a colony. Asexual reproduction in 
the lower forms is by isogametes. Com- 
pare with Fig. 325. (After Kofoid) (x 250) 

Fig. 325. Merrillosphaera africana 
This is one of the higher members of the 
family Volvocaceae, in which a colony con- 
sists of: many cells and in which sexual 
reproduction is by the union of eggs and 
sperms. Within the mother colony are two 
large daughter colonies. The daughter at 
the left contains twenty-three eggs and 
two groups of sperm cells, which are at the 
right and left. The daughter at the right 
contains three asexual granddaughter col- 
onies. (After W. R. Shaw) 
