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 
