PROCESSES RELATED TO SEXUAL REPRODUCTION 221 



Processes Related to Sexual Reproduction in Metazoa 



Sexual Differentiation. — The males and females of many 

 familiar animals are distinguishable by characteristics that 

 may not be directlj^ connected with the reproductive organs. 

 There are, however, many other animals, like starfish and sea- 

 urchins, the jelly-fish and polyps, the sponges, and many of the 

 worms, in which there are no such characteristics by which the 

 sexes may be recognized. In some instances, even ovaries and 

 testes are wanting, and the sex cells are distributed through- 

 out the bod}^ as in sponges. It is evident, therefore, that sexual 

 differentiation in the metazoa consists primaril}- in the produc- 

 tion of male and female reproductive cells which arc comparable 

 with the microgametes and macrogametes of protozoa. The 

 sexes are separate in the great majority of animals; but cases ot 

 hermaphroditism, or the production of both male and female 

 gametes by the same individual, are not infrequent among certain 

 classes. In the higher multicellular animals, the underlying 

 maleness and femalcness, which is indicated by the sex cells, may 

 be so extended to the structure and functions of the body that sex 

 seems a matter of somatic organization. Nevertheless, the pri- 

 mary factor of sexual differentiation in metazoa, as in protozoa, is 

 the production of male and female gametes. 



From an evolutionary standpoint, sexual differentiation prob- 

 ably originated in the divison of labor between gametes, whereby 

 there came to be male cells, or microgametes, which were small 

 and active with special powers of locomotion; and female cells, or 

 macrogametes, which were large, inactive, and food-laden. The 

 course of such an evolution is indicated by the gametes of protozoa, 

 which illustrate all the stages from isogamy to a very specialized 

 anisogamy (cf. p. 174). If the original forms of life were uni- 

 cellular organisms, it is, therefore, probable that sex originated 

 even before the appearance of multicellular animals, in the manner 

 suggested by the comparison of cellular cycles (Fig. 110). Among 

 metazoa, the primary sexual differentiation of male and female 

 cells has been variously complicated by the evolution of repro- 

 ductive organs and systems, which are known as -primary sexual 

 characters- and by the appearance of secondary sexual characters, 

 which are not directly related to the reproductive organs, although 

 they are expressions of the underlying maleness or femaleness of 



