THALLOPHYTES 



2') 



each one of which produces a single very large egg, conspicuously filled 

 with reserve food, and develops a perforation which the sperms enter. 

 Still other cells of the filament divide, the daughter cells not elongating, 

 thus producing a short row of small cells, the antheridia, within each 

 one of which one or two sperms are developed (fig. 77). The sperms 

 are much smaller than the zoospores, but they have the same crown of 

 cilia, and this evident relationship between spore and sperm is constantly 



Figs. 74-76. — Ocdogonium: 74, large zoospore forming within the cell, including all 

 the contents; 75, zoospore escaping from cell; 76, zoospore freed from its membrane. 

 — 74, after Pringsheim; 75, 76, after Hirv. 



appearing. In this case the oogonia and antheridia are distinct from 

 the vegetative cells, but still they are transformed vegetative cells. The 

 sperms escape from the antheridia, swarm about the oogonia, enter 

 them through the perforation, and fertilize the eggs. Although several 

 sperms may enter an oogonium, only one is concerned in the act of fer- 

 tilization, the essential feature of which seems to be the fusion of the two 

 nuclei. The oospore is a heavy- walled cell, which upon germination 

 produces four zoospores, each one of which gives rise to a new filament 

 (figs. 80-82). 



In the form of Oedogonium just described, the oogonia and antheridia 

 occur in the same filament, but in certain species they occur on different 



