GREEN ALGAE 



371 



In Bryopsis the size is unequal (ii.) ; while Codhim shows still greater 

 inequality (iii.). 



In Vaucheria, Fig. 2 75 (»v.), which is the most advanced of all in sexual 

 differentiation, the large non-motile egg, retained in the oogonium, 

 is fertilised by a small spermatozoid. 

 The facts suggest again a progression 

 from isogametes to a distinction of 

 egg and spermatozoid, but in a series 

 of Algae quite distinct from those 

 previously described. 



The sex-organs of VaucheHa arise 

 close together as short lateral branches 

 (V. sessilis), or borne together on the 

 same branch (V. terrestris, Fig. 276). 

 The male, or antheridia, are horn-like 

 curved bodies ; the female, or oogonia, 

 are oval. In the antheridium a sep- 

 tum cuts off the multi -nucleate pro- 

 toplast from the parent tube : each 

 nucleus becomes the centre of a 

 spindle-shaped spermatozoid ; and 

 these escape, with their paired cilia 

 pointing fore and aft, through an 

 opening at the distal end. The 

 oogonium also at first contains 

 numerous nuclei embedded in protoplasm stored with many globules 

 of oil. But, as the ovum matures, all the nuclei but one wander 

 back into the parent filament, which is then shut off from the oogonium 

 by a wall. Meanwhile a beak has formed at the distal end of the oogon- 

 ium, which then opens by swelling of the wall, a portion of the colour- 

 less contents being emitted. The uninucleate egg then lies open for 

 fertilisation by the small motile spermatozoids (Fig. 275, iv.). It 

 appears that self-fertilisation from an adjoining antheridium is the 

 rule. The fusion of the two nuclei has been observed to form the 

 nucleus of the zygote. Then follows storage of further oil, a change of 

 colour of the contents, and the formation of a thick wall. In this 

 state, as an oospore, a period of rest follows. Germination takes place 

 by rupture of the thick wall, and the direct formation of a new filament 

 from the contents. 



In such organisms as these it has been found that there is no obligatory 

 succession of events in the life-history. It lies in the hand of the experimenter 



Fig. 276. 



Sexual branch of Vaucheria terrestris, 

 bearing distally a curved male gametan- 

 gium or antheridium as it is calLd : and 

 right and left oval oogonia. 



