THALLOPHYTA odo 
than two cilia, are smaller than zodspores, and are produced in 
larger numbers in a cell (Fig. 328). The gametes fuse in pairs 
to form zygospores, each of which becomes surrounded by a cell 
wall (Fig. 328). After a period of rest the zygospore germinates 
and produces a small, single-celled plant, the protoplast of which 

Fig. 328. Ulothriz 
Left: above, the formation of zodspores ; below, germination of zodspores. Right: 
upper line, formation and escape of gametes; middle line, conjugation of gametes ; 
lower line, germination of gametes with the reproduction of zodspores. (Redrawn 
after Dodel-Port) 
divides to form a number of zoédspores, which, like other zo6- 
spores, grow into ordinary vegetative plants (Fig. 328). Under 
certain conditions gametes may germinate without fusion. 
The fact that the gametes are similar to zodspores and that 
gametes may germinate without fusion indicates, as in the case 
of Chlamydomonas, that gametes are modified zodspores and 
that sexuality may have resulted from a change of nonsexual 
zoospores into sexual gametes. 
