THE LIFE CYCLE 339 



The principal relations that the sexual and asexual 

 methods may bear to each other are diagrammatically 

 indicated in figure 191. Six generations are represented 

 in the six vertical columns. A small circle represents the 

 egg; a dash with a tail, the sperm; a circle inclosing a dot, 

 the zygote ; the black dots are spores, and the black 



FIG. 191. Diagram of types of reproduction; T, normal sexual reproduc- 

 tion; z, parthenogenesis; j, alternation of generations; 4, polyembryony, etc.; 

 j, occasional production of sex cells in a series of spore forming individuals; 

 6. continuous spore formation. 



dashes are egg fragments. The top line of figures repre- 

 sents ordinary sexual reproduction, occurring alone, sub- 

 stantially as previously shown in figure 174. The second 

 line represents parthenogenesis (with only three genera- 

 tions of females included between the bisexual generations*). 



*In most parthenogenetic species, the sexual generations recur 

 at much greater intervals. In fact, in certain species of rotifers 

 and also in certain gall wasps (Cynipidae) no males are known, 

 and parthenogenetic reproduction appears to be continuous. On 

 the other hand in one genus of gall wasps (Neuroterus) single 

 sexual and parthenogenetic generations regularly alternate; and, 

 strangely enough, the females of the latter differ so much in form 

 and structure from the former that they have been described as 

 a different genus (Spathegaster) . 



