198 STRUCTURAL BOTANY 



ova are in pairs, held together by the inner oogonial wall. 

 There are many packets of spermatozoids and many pairs 

 of ova sent out from each conceptacle. As the spray 

 dashes up over the plants with the returning tide, their 

 reproductive cells are washed down from the receptacles, 

 sometimes onto the rocks, sometimes only onto the lower 

 part of the plant itself, where they often come to rest in 

 the groove of the thallus. 



It is a constant rule among the Fucacese that 

 fertilisation takes place outside the parent plant, after 

 the sexual cells have been set free. The remaining 

 antheridial membrane bursts after expulsion from the 

 conceptacle, and the spermatozoids are at liberty to 

 swim off by means of their cilia (Fig. 84, B). In 

 this species, however, the ova remain enclosed within the 

 soft mucilaginous membrane derived from the oogonial 

 wall. In most Fucacese this is not the case ; the ova 

 are set free as bare masses of protoplasm ; the peculiar 

 state of things in Pelvetia probably has to do with the 

 long exposure to the air; the mucilaginous envelope 

 protects the protoplasm within from danger of drought. 



The spermatozoids during their movements come 

 across the pairs of ova, and swarm around them in large 

 numbers. Some of them make their way into the 

 mucilage, and penetrate to the protoplasm, which it seems 

 is generally approached at the side where the two ova 

 are in contact. It has been shown that ultimately only 

 a single spermatozoid succeeds in entering the protoplasm, 

 and making its way to the nucleus of the ovum. The 

 details of the process have now been thoroughly worked 

 out, and the fusion of the small male nucleus with the 

 large nucleus of the ovum observed, as shown in Fig. 87, 

 which represents the act of fertilisation in another member 



