I THE PROBLEM 5 



plant emerge, become mature, and themselves ripen germ 

 cells which give rise to a new generation. 



Whatever the details of development, one cardinal 

 fact is clear. Except for the relatively rare instances 

 of parthenogenesis a new individual, whether plant or 

 animal, arises as the joint product of two sexual cells 

 derived from individuals of different sexes. Such sexual 

 cells, whether ovules or ova, spermatozoa or pollen grains, 

 are known by the general term of gametes, or marrying 

 cells, and the individual formed by the fusion or yoking 

 together of two gametes is spoken of as a zygote. Since a 

 zygote arises from the yoking together of two separate 

 gametes, the individual so formed must be regarded 

 throughout its life as a double structure in which the 

 components brought in by each of the gametes remain in- 

 timately fused in a form of partnership. But when the 

 zygote in its turn comes to form gametes, the partnership 

 is broken and the process is reversed. The component 

 parts of the dual structure are resolved, with the formation 

 of a set of single structures, the gametes. 



The life cycle of a species from among the higher plants 

 or animals may be regarded as falling into three periods : 

 (i) a period of isolation in the form of gametes, each a liv- 

 ing unit incapable of further development without inti- 

 mate association with another produced by the opposite 

 sex; (2) a period of association in which two gametes 

 become yoked together into a zygote and react upon one 



