THE BRYOPHYTA 119 



in the canal, arising from the disorganised neck canal- 

 cells. This substance takes up water, swells, and so 

 forces the four cap-cells apart, causing the neck to open, 

 while at the same time a portion of the mucilage pro- 

 trudes through the opening. 



c. Fertilisation 



In Pellia, as in the Cryptogams generally, fertilisation 

 must take place under water ; after rain or dew the 

 surface of the thallus is wet enough for the spermato- 

 zoids to accomplish their journey. The cells of the 

 antheridial wall take up water, swell, and press upon 

 the mass of spermatozoid mother-cells. The antheridium 

 bursts, and its contents are set free. As soon as the 

 spermatozoids are released from their mother-cells, they 

 swim through the water, rotating as they go, in much 

 the same way as those of a Fern. They are also drawn 

 towards the archegonia as soon as they come within 

 their " sphere of influence," but in this case the chemical 

 nature of the attractive substance has not yet been 

 determined. They are caught in the mucilage, wriggle 

 down the neck of the archegonium, and one of them 

 effects fertilisation by union with the nucleus of the 

 ovum. In all this process, so far as the details have 

 been worked out, there is exact agreement with the 

 Vascular Cryptogams. 



So much the more surprising is the remarkable 

 difference in the ultimate product of fertilisation. The 

 ovum when fertilised surrounds itself as usual with 

 a cell - wall, and begins to divide. The result of 

 this development will be considered in the next 

 section. 



