246 



STRUCTURE AND LIFE HISTORIES 



enters it, and makes its way through the cytoplasm to the 

 egg-nucleus (Fig. 181), with which it fuses, thus completing 

 the act of fertilization. The other sperms come to rest 

 outside the egg, and finally disintegrate. If eggs liberated 

 at different periods occur in the water together, the sperms 

 will swim toward those liberated last, in preference to the 

 others. As soon as fertilization has been accomplished, 

 the oosperm begins to divide, and develops into a new 

 plant , which finally comes to resemble externally the ones 

 that bore the antheridia and oogonia. 



FIG. 181. Dictyota dichotoma. At the left, section of newly liberated 

 egg; e, egg; cyto, cytoplasm of egg; sp, three of the numerous sperms ap- 

 proaching the egg to fertilize it. At the right, portion of a section of an 

 egg after one of the sperms (shown at the right of the egg-nucleus, en) has 

 entered; sp, sperms that have not entered. The fusion of the sperm with 

 the egg nucleus has been delayed, and the sperm has greatly increased in 

 size. (Redrawn from J. Lloyd Williams.) 



232. Asexual Reproduction. The plants that develop 

 from fertilized eggs never bear antheridia and oogonia, but 

 non-motile, asexual spores only. These are set free at 

 irregular intervals, and never at rhythmic periods like the 

 gametes. They are produced by two successive divisions 

 of a spore-mother-cell, and thus occur in groups of four 

 (tetrads) ; the plants that bear them are commonly referred 



