THALLOPHYTES 



27 



gametangium that produces an egg (usually only one) is 

 an oogonium (" egg-case "), while the gametangium that 

 produces sperms is an antheridium (a name whose meaning 

 explains nothing). 



There are two distinct stages in the evolution of oogonia 

 and antheridia that ought to be recognized. In (Edogonium 

 (Fig. 15, B), for example, the oogonia and antheridia are 

 transformed vegetative 

 cells ; that is, cells which 

 do vegetative work may 

 later become oogonia 

 or antheridia. But in 

 Vaucheria (Fig. 21) and 

 Fucus (Figs. 18, 19), for 

 example, the oogonia and 

 antheridia have never 

 been a part of the vege- 

 tative body, but are set 

 apart from the beginning 

 as special branches. In 

 these forms, therefore, 

 the sex-organs have be- 

 come completely differ- 

 entiated from the rest 

 of the body. 



20. Differentiation of sex-individuals. -- There is another 

 kind of sexual differentiation that must be recognized. In 

 such Algae as Spirogyra (Fig. 22), the two gametes look alike, 

 both of them being large, but one of them remains passively 

 in its cell, while its mate leaves its cell, squeezes through a 

 connecting tube, and enters the other cell. The behavior 

 of the two gametes, therefore, is different, and it seems 

 proper to call the passive one female and the active one 

 male. Furthermore, it is very common to find two fila- 

 ments of Spirogyra lying side by side, all the opposing cells 



FIG. 19. Fucus: a cummuer in the body in 

 which oogonia are produced. After THURET. 



