THE ALG.E 



231 



gamete, all but one of the nuclei disappear, and a projecting 

 beak is formed, filled with clear cytoplasm free from green 

 chloroplasts. With these changes the gametangium and its 

 inclosed gamete are made ready for the reception of the male 

 gamete at the time of fertilization. The male gametangium, 

 which began as a cylindrical outgrowth of the reproductive 

 branch, elongates and cuts off an end cell, which forms the male 

 gametangium (6). The dense granular cytoplasm of -the male 

 gametangium, instead of forming a single gamete, as in the 



igomum 



or female 

 gamete 



'ell wall 

 toplasm> 



Anther idiu 

 Male, 



Female ga 

 branch 



FIG. 120. Sexual reproduction in Vaucheria 



a, portion of a filament with male and female reproductive branches; b, antheridium 



and oogonium, with sperms and egg ; c, liberation of sperms, and sperms swarming 



around the receptive spot of the egg ; d, sperms ; e, fertilized egg, with union of 



male and female pronuclei 



female, divides into a very large number of naked protoplasts, 

 each of which becomes a motile male gamete. These minute 

 male gametes are finally liberated in the water by the softening 

 and rupture of the cell wall at the end of the male game- 

 tangium (e) and are ready to effect a union with the eggs in 

 the female gametangia. 



Fertilization results from the union of the cytoplasm and 

 nucleus of one of these small male gametes with that of the 

 large female gamete (Fig. 120, c). In order to effect this 

 union the cell wall at the end of the beak of the female game- 

 tangium softens and (e) finally breaks down so that a free passage 

 is made for the motile gamete to the cytoplasm of the female 



