iS SEX AND HEREDITY 



Seaweeds : being pear-shaped, with two cilia attached 

 laterally. The female gamete acts attractively upon the 

 smaller actively motile males, which collect round it 

 (Fig. 8, iv, v). Finally one fuses with it ; it then retracts 

 its cilia, settles, and germinates. Here then is a step 

 in advance of E. siliculosus. The sexual distinction is 

 not merely functional, but is marked by difference of 

 size, and that difference can already be seen in the spor- 

 angia that give rise to the gametes (gametangia) . 



A further step in the distinction of sex appears in 

 Cutleria, an Alga with a narrow thong-like frond, which 

 bears gametangia of two distinct sorts. The one kind is 

 small-celled, and produces a small male gamete from each 

 cell. The other is larger-celled, and produces large female 

 gametes. Both, though differing greatly in size, have the 

 characteristic pear-like form with two cilia attached 

 laterally ; and at first both are motile. But soon the 

 larger female gametes retract their cilia, and lose their 

 motility, and round off to a sphere, with a clear receptive 

 spot. The male gamete which retains its motilit}^ is 

 attracted to it, and fuses with it as before (Fig. 9). The 

 points of advance here are the greater difference in size of 

 the gametes, and the loss of the motility and rounding off 

 of the female before fertilization. \Ye may now distinguish 

 the smaller male gamete as a spermatozoid, and the larger 

 female gamete as an ovum, or egg. 



The next step is illustrated in Fucus, in the fact that the 

 large spherical female gamete, or egg, is never motile at all, 

 while the small spermatozoid retains its motility. The 

 large leathery frond of the Common Wrack is fertile at 

 the ends of some of its branches. Flask-shaped cavities 

 are there borne (conceptacles), and they contain the 

 sexual organs. In some species they are both borne 



