REPRODUCTION 431 
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. These free-swimming similar sexual cells are 
frequently called planogametes. 
In the Zygnemece 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 awhile 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 Zygnemew, 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 distinguished 
in the Mesocarpew, where both gametes meet in the connect- 
ing passage. 
In Ulothriz the differentiation of sex is even more rudi- 
mentary, 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 fact suggests 
that the sexual cells have been derived from asexual ones, 
and are a later development, therefore, in the history of the 
race. 
ws 
