28 



INBBEEDING AND OUTBBEEDING 



think of the stationary cell as female and the other 

 as male. 



Another line of development, however, became the 

 dominant one in the plant kingdom just as it did in the 

 animal world. A morphological differentiation of the sex 

 cells occurred. One became a large inactive cell stored 



FIG. 6. Ulothrix, a primitive type of sexual reproduction. A, B, filaments; C, zo- 

 ospores; D, germination of zoospore; E, gamete formation in filament; F, gametes and their 

 fusion; (?, germination of zygospore. (Bergen and Davis after Dodel.) 



with food, the egg; the other became small and motile, the 

 sperm. This change is well illustrated in Fucus (Fig. 5), 

 one of the brown algae. It is clear that such a change in- 

 creased the probability of fertilization, since many 

 sperms could be produced without utilizing a great deal 

 of energy, and since the attraction of the egg for the sperm 

 was presumably augmented. 



A further stage in the evolution of sex was reached 

 when the cells producing the eggs or the sperms were 



