174 



VAUCHERIA AND CAULERPA 



(p. 132) : the large inactive egg-cell which furnishes by far 

 the greater portion of the material of the oosperm is the 

 female gamete ; the small active sperm-cell, the function of 

 which is probably (see Lesson XXIV.) to furnish additional 

 nuclear material, is the male gamete. 



Similarly the oosperm is evidently a zygote, but a zygote 

 formed by the union of the highly differentiated gametes, 



FIG. 39. Caulerpa scalpelliformis ( nat. size), showing the stem- 

 like, root-like, and leaf-like portions of the unicellular plant. (After 

 Harvey. ) 



ovum and sperm, just as a zygospore (p. 164) is one formed 

 by the union of equal sized gametes. 



As we shall see, this form of conjugation often distin- 

 guished as fertilization occurs in a large proportion of 

 flowerless plants, such as mosses and ferns (Lessons XXVIII. 

 and XXIX.), as well as in all animals but the very lowest. 

 From lowly water-weeds up to ferns and club mosses, and 

 from sponges and polypes up to man, the process of sexual 

 reproduction is essentially the same, consisting in the conju- 

 gation of a microgamete or sperm with a megagamete or 



