THALLOPHYTA: ALGAE 



83 



produce two sperms. The sperms escape from the antheridium in a 

 mass surrounded by a membrane that soon disappears. 



The eggs of Fucus are borne in groups of eight inside the oogonia, which 

 are large oval or globular cells, each of which has a one-celled stalk (Fig. 

 71B-D). The young oogonium has a single nucleus, the division of which 

 is reductional. Three simultaneous divisions result in the formation of 

 eight free nuclei. Cytoplasmic cleavage follows and an egg is organized 

 around each nucleus. The eggs are extruded from the oogonium in a 



A -^""-^ D ' ^^- E 



Fig. 71. Sex organs oi Fucus furcatus. .-i, antheridial filament, X 320; B, young oogonium 

 with four nuclei, X160; C, longitudinal section of an older oogonium with eight nearl,\' 

 mature eggs, X160; D, mature oogonium, X160; E, escaped egg of Fucus vesiculosus sur- 

 rounded b>' numerous sperms, X 240. (E, after Thuret.) 



group surrounded by a membrane that soon ruptures. In Ascophylhun 

 four eggs are organized in an oogonium, in Pelvetia two, and in Sargassum 

 only one. In all the genera of Fucales, however, eight nuclei always arise 

 in the oogonium, the nonfunctional nuclei either being extruded or 

 degenerating. Thus Fucus represents the primitive condition from which 

 the other genera, by progressive reduction, have been derived. 



Depending on the species, the antheridia and oogonia of Fucus may 

 occur in the same conceptacle, in different conceptacles on the same plant, 

 or on different plants. In addition to the sex organs, the conceptacles 

 contain numerous unbranched sterile filaments (paraphyses), some of 

 which often project through the pore. Both the sperms and eggs escape 

 from the conceptacles into the water but only the sperms are motile. The 

 sperms surround the eggs in such vast numbers that they cause them to 

 rotate (Fig. HE). After fertilization has taken place, the zygote sur- 



