ONTOGENESIS 167 



times until two hundred or more result, each of which 

 forms a " spore-mother cell," which, in turn, pro- 

 duces a number of spores. From each spore there 

 develops a new Amoeba. 



Parthenogenesis. We have seen that reproduc- 

 tion is a kind of discontinuous growth, and that 

 while it is a usual accompaniment of zygosis in the 

 higher forms, there is probably no fundamental con- 

 nection between the two phenomena. Zygosis, 

 whether partial or incomplete, may perhaps be looked 

 upon as a sort of rejuvenescence of the organism which 

 is the more necessary and occurs the more fre- 

 quently in proportion to the degree of specialization 

 of the organism. In the majority of the higher 

 forms it is necessary with every individual generated. 

 Some of the Me.tazoa, however, like the Protozoa, are 

 able to produce successive generations for a long 

 time without the stimulus of sexual conjugation. 

 Such a phenomenon is known as parthenogenesis or 

 agamy, and occurs as a normal incident of existence 

 in many forms, particularly the insects. 



In the plant-louse, or Aphis, the outline of the life 

 history of a common species is something as follows : 

 in the autumn the female lays a large so-called winter 

 egg, which, like most eggs, has been " fertilized " by 

 union with the sperm. In the following spring this 

 egg hatches into a wingless female called the stem- 

 mother, that forthwith begins to reproduce partheno- 

 genetically a long series of generations like herself, 

 precisely as a single Paramecium may populate a 



