REPRODUCTION 17 



the body thus formed cannot, however, continue to divide 

 indefinitely, but sooner or later reach their full develop- 

 ment and do not thereafter increase in bulk by further 

 division. 



18. Sexual reproduction. Reproduction in the higher 

 forms of organic life is a complicated process and is ac- 

 complished by the activities of a special group of cells 

 which is called the reproductive or generative system. 

 These germ-cells are highly specialized, and their develop- 

 ment and functions divide the individuals of a species 

 into two sexes, male and female. The two sexes differ 

 widely in form and characters, but particularly in respect 

 to their physiological relations to reproduction. The 

 product of the male germ-cells is the spermatozoon, an 

 exceedingly small cell but carrying in its minute substance 

 the inherited potentialities of its ancestors. The female 

 germ-cell or ovum similarly derived from the female is 

 much larger, but also carries the germ substance which 

 later, after fertilization by the spermatozoon and under 

 proper conditions, will become a new individual. 



19. The reproductive process. The essential fact 

 in the reproductive processes of all vertebrate animals 

 is the formation of an egg or ovum by the female and of a 

 fecundating fluid containing the spermatozoon or sperm- 

 cell by the male. The union of the two germ-cells is the 

 first step in the independent existence of the new indi- 

 vidual. The conjugation of the egg and spermatozoon 

 sets in motion a series of events which, whether viewed 

 from the standpoint of racial significance or of biological 

 interest, has no parallel in the whole realm of biology. 

 Upon the successful development of the fertilized egg- 

 cell depends the future of the species. The union of the 

 egg and spermatozoon takes place in some forms outside 



