The Lower Plants 67 



ited in a most instructive fashion the gradual 

 evolution of the sexual reproductive cells. It is clear 

 from a study of these that this evolution has arisen 

 quite independently in several widely separate lines, 

 but the course of evolution is extraordinarily similar 

 in all of these. The Volvocales and the Ulothri- 

 cales, among the green algae, are excellent exam- 

 ples of this, and the brown algae also show all 

 stages of evolution of the gametes or sexual cells, 

 from perfectly similar ones, hardly distinguishable 

 from the non-sexual zoospores, to clearly differen- 

 tiated small male cells or spermatozoids and large 

 non-motile female cells or eggs ( Fig. 5 ) . 



The difference between the lowest type of sexual 

 cells and the non-sexual zoospores is very slight, and 

 there seems no question that the gametes were orig- 

 inally derived from cells capable of development 

 without fertilization. Fertilization in its simplest 

 form consists in the union of two complete and per- 

 fectly similar cells, the union extending to the 

 fusion of the nuclei into one, and also possibly the 

 fusion of the chromatophores. In the course of de- 

 velopment the two gametes become more and more 

 dissimilar, one diminishing in size but usually re- 

 maining actively motile, the other becoming larger 

 and losing the power of motion. The first is the 

 male gamete or sperm, and is largely composed of 

 the nucleus of the mother cell ; the large, passive 

 cell is the female cell, egg, or ovum. Among the 

 lower types the gametes may sometimes germinate 



