KEPKODUCTION 481 



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 Mesocarpece 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 Zygnemece, 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 Mesocarpece, where both gametes meet in the connect- 

 ing passage. 



In Ulothrix 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. 



