14 SEX AND HEREDITY 



two cilia (Fig. 6, D). They are gametes, or sexual cells. 

 It has been observed that if gametes escaping from different 

 filaments meet, they coalesce in pairs (Fig. 6, E, F, G). 

 They show that type of syngamy called conjugation, in 

 which the two sexual cells are alike in form and structure, 

 though of distinct origin. There is here no differentiation 

 of sex. It is impossible to distinguish one as male and 

 the other as female. The result of the fusion is called 

 the zygote. It soon retracts its cilia, settles and grows, 

 and after a period of rest germinates, dviding its contents, 

 which then escape as zoo-spores. 



Ulothrix is an interesting instance of rudimentary 

 sexuality. It shows syngamy ; but the gametes are 

 isogametes. In form they are like the zoo-spores, except 

 in the number of the cilia. But after fusion the zygote 

 has like them four cilia. There are differences in size 

 and behaviour of the zoo-spores, some resulting from 

 repeated divisions of the parent cell, being smaller than 

 the type, though still having four cilia ; and they germi- 

 nate like them. Further, it has been seen that sometimes 

 the gametes themselves may germinate without fusing. 

 These facts have an important bearing on the theory of 

 origin of the gametes in so simple a plant. They suggest 

 that the gamete is really a zoo-spore reduced in size and 

 quality as a consequence of repeated divisions of the 

 parent cell. And that fusion, or syngamy, between such 

 gametes of a distinct source gives an impetus to new 

 development in these weakened cells. When we further 

 reflect that the motile is probably the primitive state of 

 these cells, and compare them with such an organism as 

 Euglena, it seems probable that sex may have arisen as an 

 offset against a weakening of the cells by divisions repeated 

 more rapidly than their substance is increased by growth. 



