7 

 The Fertilization-process 



C^yiLL ANIMAL AND PLANT FUNCTIONS CENTER AROUND 



two processes: nutrition and reproduction. The first is 

 concerned with the blindly egoistic struggle of the individual 

 to preserve itself. The second relates to the altruistic 

 struggle of organisms to perpetuate their kind. Despite 

 their interdependence the processes of nutrition and repro- 

 duction have different values for the organism. Without 

 food or the apparatus for the utilization of food, the organ- 

 ism dies; without the reproductive apparatus (as sys- 

 tems, organs, tissues or cells) sexual organisms can still 

 live. The reproductive (germ) cells are sharply set off 

 from all other (somatic) cells; they have the special burden 

 of the perpetuation of the species. The sex-cell is therefore 

 a thing apart, a tenant housed by mortal somatic cells and 

 like them mortal while the tenancy lasts. House and ten- 

 ant have a common origin. Their separation constitutes 

 the first of the series of those differentiations that mark 

 the development of an individual animal. 



The adult animal is derived from one cell, the egg, which 

 by the process of cell-division becomes a mass of cohering 

 cells. They form the germ-layers; these give rise to the 

 various organs (or systems of organs) which compose the 

 complex adult individual. In the course of this develop- 

 ment are set off from all the other cells which make up the 

 body of the animal certain ones, the primordial germ-cells, 

 whose function is to produce either eggs or spermatozoa in 

 bisexual animals or both of these in monosexual or herm- 

 aphroditic animals. Thus, the germ-cells, first differenti- 



147 



