422 VEGETABLE PHYSIOLOGY 
produced in larger numbers and have only two cilia each 
(fig. 169). After they are set free into the water they 
swim about for some time, and then they usually fuse 
together in pairs, nucleus joining nucleus and protoplasm 
uniting with protoplasm. The new body so formed is 
known as a zygospore. After a period of rest it can give 
rise to a new filament. As there is no difference between 
the cells which unite to form this structure, they are 
frequently called nlanogametes. 
In the Zygnemee and the Mesocarpee the gametes are 
solitary and non-motile and do not escape from the cells 
in which they are formed. Two filaments take part in 
the fusion of the gametes; these are found lying close 
together in the water; from a cell of each filament a 
protrusion grows out towards the other and the two come 
into contact and join, the separating walls breaking down. 
The contents of one cell pass over into the other through 
the channel so formed, or the contents of both the cells 
meet in the middle of the passage; fusion of the two takes 
place, and the new body, called as before the zygospore, 
clothes itself with a cell-wall. It is liberated after a while 
by the breaking down of the wall of the structure which 
encloses it, and can then give rise to a new individual. 
A similar process is characteristic of certain Fungi. 
In all these cases, though the cells are sexual cells, 
the differentiation of sex is so slight that it is difficult to 
speak of male and female gametes. In the Zygnemea, in 
which the formation of the zygospore takes place in the 
cell of the filament, the gamete which passes through the 
passage may perhaps be regarded as male and the more 
passive one as female. This differentiation cannot be dis- 
tinguished in the Mesocarpee, where both gametes meet 
in the connecting passage. 
In Ulothriz the differentiation; of sex is even more 
rudimentary, as it is not always necessary for the fusion 
to take place. If any cell escapes fusion it may develop 
into a new filament independently of this process. This 
