200 EVOLUTION, GENETICS, AND EUGENICS 



The term "zygote" is also sometimes used to designate the organism 

 that develops from the united gametes. 



Gametic reproduction is very frequently evaded in both animals 

 and plants by the use of a process known as parthenogenesis, or "virgin 

 birth," in which eggs develop without any aid from sperms. Some 

 instances of this peculiar aberration of gametic reproduction will be 

 discussed in a special chapter on the biology of sex (chap. xvii). Here 

 we have the anomalous situation of gametic (or marrying) reproduction 

 without the presence of both sexes. Long-continued parthogenesis 

 involves the reduction of diversity and results in constancy as marked 

 as that which is associated with various forms of sexless reproduction. 



While the great majority of animals and plants are sexually di- 

 morphic, consisting of male individuals that produce sperms and fe- 

 male individuals that produce eggs, large numbers of both animals and 

 plants are monoecious, having both sexes present in one individual. 

 Such forms among animals are known as hermaphrodites. In some her- 

 maphrodites eggs are fertilized by own sperms, in others mating occurs 

 in which a mutual exchange of sperms takes place. In the latter in- 

 stances the advantages of sex in enhancing diversity are retained, and 

 mating is facilitated because any two adult individuals may mate; but 

 in the former instances where self-fertilization occurs, diversity is re- 

 duced to the level found among organisms reproducing by asexual 

 methods. It is possible, therefore, to produce from such self-fertiliz- 

 ing monoecious species pure lines in which all progeny of a single in- 

 dividual are genetically identical. In a later connection we shall make 

 use of such forms as these to study heredity in its simplest expression 

 (chap. xv). 



THE ORIGIN OF GAMETES 



Two important questions arise in connection with gametes: (a) 

 From what cells in the parent body do gametes arise? (b) What 

 changes occur in germ cells that make it possible for them to unite in 

 pairs to form zygotes? 



The germ track. — The question as to whether germ cells are derived 

 from parental tissues that have been more or less specialized for other 

 functions, or whether they are derived from an unbroken series of 

 germ cells set apart from bodily functioning at all times, is one of great 

 importance for theories of heredity. The prevailing view of biologists 

 is that, at least in the higher animals, and possibly in all animals and 

 plants, the germ cells are produced from cells that have never in any 



