THE ALG^E 



199 



of the Fucaceae. The proof of sexuality has also been 

 afforded by experiment. If the ova are kept apart from 

 the spermatozoids (as can be easily done in the case of the 

 dioacious species) they soon perish, making perhaps some 

 slight attempt at germination, which comes to nothing. 



FIGS. 86, A and B. A, oospores o* Pelvetia beginning to divide 

 immediately after fertilisation. They are still enclosed in 

 the gelatinous inner wall of the oogonmm. The small 

 bodies in the gelatinous mass are spermatozoids ; the larger 

 bodies near the septum are the rejected nuclei of the 

 oogonium. B, later stage. The oospores have divided 

 to form many cells, and are sending out rhizoids. Magnified 

 about 120. (After Thuret and Bornet.) 



If, however, the spermatozoids have access, the result is 

 quite different. The ovum now surrounds itself with a 

 cell- wall of its own, and after a few hours begins to divide. 

 The direction of the first cell- wall formed across the 

 fertilised ovum is said to be determined by light, and to 

 be always at right angles to the incident rays. Other 



