THALLOPHYTA— ALG^ 



55 



F:g. 803. 



a number of bi-ciliated gametes, which are liberated by rupture 

 of the cell-wall. These, after a period of activity, conjugate in 

 pairs, the conjugating cells generally originating from two 

 different cells or even two different filaments. The differentia- 

 tion of sexuality in some of these is very slight, for if they fail to 

 conjugate they can still germinate as if asexual cells. In 

 another group, while apparently any ceU of the filament can 

 become a gametangium, the contents are not differentiated into 

 oihated free-swimming bodies, but the whole of the proto- 

 plasm undergoes a kind of rejuvenescence and forms a single 

 gamete. In other forms definite antheridia and oogonia are 

 produced, which furnish antherozoids and oospheres respect- 

 ively. In each oogonium, as a rule, only a smgle oosphere 

 is produced, and this is fertilised in situ by the entry of an 

 antherozoid into the 

 oogonium [fig- 805) . 

 This marks a striking 

 advance upon the sex- 

 ual process in the 

 Phseophyeese, where 

 the oospheres are set 

 free and fertihsed out- 

 side in the water. It 

 approaches to the con- 

 dition in the next 

 group, where more 

 elaborate provision is 

 made to bring the 

 sexual cells together. 



In the cases of conjugation of the solitary gametes in a game- 

 tangium, the fusion of the cells always occurs within one of 

 the cells or an outgrowth from it, the gametes never being set 

 free. 



In the latter case parthenogenesis sometimes occurs, a gamete 

 developing into a new filament without any process of conjuga- 

 tion. Such gametes are called azygospores. 



In one exceptional case, Sphceroplea, several oospheres are 

 formed in the oogonium, which is not a specially differentiated 

 cell. Here, too, fertihsation takes place internally, antherozoids 

 finding their way into the cell by an opening in the wall. 



Asexual cells are produced by most, but not by all the mem- 

 bers of this group. They are generally free-swimming cells, 

 or in some cases ccenocytes, either ciliated all over their surface. 



Fig. 803. Part of a filament of Ulothrix from which 

 the gametes, j/, are escaping. g\ Free gamete. 

 g~, g^. Gametes conjxigating. 



