THE ALGAE 



119 



B 



Fertilization 



Both kinds of gametes are discharged at any time of the day or night, 

 though it has been found in the Mediterranean that they are most abundant 

 between 5 and 7 a.m. 



The male gamete is considerably smaller than the female (Fig. loi) ; it 

 possesses two unequal flagella like those of the female, and a red pigment 

 spot, but few or no plastids. y 



It mav remain active for as 

 long as twenty hours. The 

 active male gamete usually be- 

 comes attached to a female 

 gamete only when the latter 

 has become sluggish, but be- 

 fore it has produced any en- 

 veloping wall. The flagella of 

 the male gamete are then with- 

 drawn, and the cytoplasms of 

 the two cells coalesce and 

 fusion of the two nuclei 

 follows. 



Should a female gamete 

 fail to be fertilized it is still 

 capable of growth, though only 

 after becoming enclosed in a 

 cell wall. A period of rest 

 usually follows before any 

 division takes place. 



Compared with the rate of 

 cell division in the fertilized zygote the rate of development of these 

 apogamous sporelings is very slow. 



Fig. ioi. — Cutleria multifida. A, Male gamete. 

 B, Female gamete. C, Zoospore. {After 

 Yamanouchi.) 



Development of the Zygote 



The development of the zygote and of the apogamous female gamete is 

 similar. The cell first elongates, forming a spherical apex attached to a 

 filamentous lower end. Division follows cutting off the spherical apex ; 

 repeated transverse divisions of the apical region follow, so that by the time 

 the sporeling is about ten days old it consists of a columnar structure standing 

 upright on the substratum with a stalk composed of one or two cells and a 

 head made up of three or four cells. The direction of growth now changes 

 and there is formed a flat plate of tissue derived from the basal end of the 

 stalk. The basal expansion grows by division of the marginal cells, which 

 causes zonation of growth to appear similar to that in Coleochaete. As growth 

 continues the outline of this flat plate of cells becomes more and more 

 irregular, and by the time it is about a month old it is large enough to be 



