THALLOPHYTES 
cell of the Volvocales, even to the " eye spot " and contractile vacuoles. 
The spores escape through an opening in the wall of the mother cell, 
swim about, come to rest, settle upon some support, ciliate end foremost, 
develop a holdfast, begin cell division, and new filaments are started 
(fig. 57). This differentiation between zoospores and ordinary vegetative 
cells, and the behavior of the zoospores in escaping individually from 
the mother cell 
and starting inde- 
pendent individu- 
als, are features in 
contrast with the 
ordinary situation 
among the Vol- 
vocales and Proto- 
coccales. In any 
event, it should be 
kept in mind that 
the zoospore type 
of cell is probably 
to be regarded as 
the most primitive 
type among the 
FIGS. 55-62. Vlothrix: 55, base of filament with holdfast; 
n algae. ^ filament producing zoospores or gametes; 57, young fila- 
The sexual re- ment developed from zoospore; 58, filament discharging zoo- 
production is isoe- s P res anc * g arnetes 5 59. an escaped zoospore; 60, escaped and 
pairing gametes; 61, zygospores; 62, zygospore producing 
amous, the ga- zoospores. 55-61, after COULTER; 62, after DODEL-PORT. 
metes being pro- 
duced just as are the zoospores, but differing in usually being smaller, and 
of course more numerous, and in being biciliate (fig. 60). The freed 
gametes conjugate in pairs and form zygospores (fig. 61), which after 
the resting period do not give rise to new filaments directly, but produce 
several zoospores (fig. 62) that give rise to filaments. This intercalation 
of zoospores in the life history between the zygospores and the new 
filaments is an important fact to note. 
Ulothrix is very often used to illustrate the origin of sex among the 
green algae. In many forms the vegetative cells are found producing 
swimming cells of all sizes, varying from the large zoospores to the 
small gametes, so that it is often impossible to distinguish between zoo- 
spores and gametes by their appearance. Some of the swimming cells 
