160 ORIGIN AND NATURE OF SEX 



in size, is called heterogamous, meaning unequal gametes. This 

 variation in the size of the gametes which we call the differentia- 

 tion of sex is due to the amount and nature of the material stored 

 in them, the male gametes being formed in a cell in larger num- 

 bers than in the case of the females. These heterogamous 

 gametes do not differ in nature from the isogamous gametes. 

 They are also lacking in some substance that renders them in- 

 capable of growth unless fusion of two gametes is effected. 



(b) Colonial Unicellular Algae. The various stages in the 

 evolution of sex are also well illustrated in several genera of 

 unicellular green algae that live together in colonies. Pandorina 

 shows the earlier phases of the differentiation of the gametes. 

 This plant is really a spherical colony of about 16 motile plants, 

 each similar to Sphaerella but held together in a mucilaginous 

 mass (Fig. 102, A). These beautiful colonies multiply by each 

 plant or cell dividing into 16 daughter cells which finally become 

 an independent colony (Fig. 102, B). The gametes are formed 

 in essentially the same way as the colonies, save that the 16 

 daughter cells finally escape as free swimming bodies (Fig. 102, 

 C). The mother plants or cells producing these gametes differ in 

 size, owing, doubtless, to the amount of nutriment which they 

 receive. Consequently, the gametes formed from them must 

 also differ in size. It is also possible that the mother plant may 

 produce a larger or smaller number of gametes and so cause a 

 variation in their size. It is interesting to note that these gametes 

 fuse quite regardless of their size, although there may be a slight 

 tendency for the small and more active gametes to fuse with 

 the larger and slower moving ones. Thus we have an illustra- 

 tion of the first appearance of the differentiation of sex although 

 the sexuality of the gametes is not thoroughly established. The 

 gametospore, resulting from the union of the gametes, develops 

 a thick cell wall, the contents assumes a reddish color and it 

 passes into a resting stage (Fig. 102, F). When conditions are 

 favorable, it germinates as shown in Fig. 102, G-H. In the 

 beautiful spherical colonies of Eudorina and Volvox (Fig. 103) 

 the differentiation of the gametes is complete. Certain cells 

 produce but one large female gamete that remains motionless 



