THALLOPHYTA 349 
Short filaments, which are branches from neighboring hyphe of 
different plants, grow toward each other, and the tips come in 
contact. The end of each hypha is cut off by a cross wall. This 
is followed by the absorption of the walls separating the two 
terminal cells, which fuse together to 
form a single cell. This grows into a 
rounded cell, surrounds itself with a 
thick cell wall, and thus becomes a resting 
zygospore. When the zygospore ger- 
minates, it bursts the thick wall and 
produces a new plant (Fig. 361). 
Relationship. The Zygomycetes are 
generally considered as having been 
derived from the Odmycetes. In the 
Odmycetes the antheridia and odgonia 
are very different in size and structure, 
while in RAzzopus the conjugating hyphe 
are similar. This difference is bridged 
to some extent by other members of the 
Zygomycetes, in which one of the conju- 
gating filaments is much larger than 
the other. 
If we consider the evolutionary tend- 
encies as shown by the Phycomycetes, it 
will be evident that they have taken a }4¢.361. Germination of 
very different direction from that ob-  zygosporeof Mucor mucedo, 
served in many of the alge. The loss of a mold related to Rhizopus 
chlorophyll by the Odmycetes, with an Modified after Brefeld 
accompanying change in the method of 
obtaining food so that the plants become saprophytes and para- 
sites, may be regarded as the initial step in a process of degener- 
ation. In the most primitive of the Odmycetes sexual reproduction 
is due to the fertilization of highly developed eggs by motile 
sperms, as is also the case in the green alga Vaucherta. In such 
forms as Saprolegnia the male cells have lost their motility, while 
in such members of the Zygomycetes as Rhizopus the fusing cells 

