THE DETERMINATION OF SEX 327 



few or no differences. The gametes of Sphaerella, for example, 

 are both motile cells of the same form, but they are not potential 

 individuals for two must unite to bring about the development 

 of a complete organism (Fig. 187). In higher organisms the 

 female gamete becomes more and more specialized as a passive 

 cell in which food for the developing embryo is concentrated, 

 while the male gamete is a highly specialized motile cell, unham- 

 pered by stored food and therefore able with maximum efficiency 

 to seek out the egg cell. The difference may be extreme, yet as 

 we have seen in Chapter XVI, the cells are potentially equal in 

 their contributions to the new generation. 



Among the more primitive organisms and among some higher 

 species, differentiation of the individuals producing the two kinds of 

 gametes is conspicuously lacking, even to the extent of hermaph- 

 roditism. In this condition the same individual produces both 

 male and female germ cells. In the higher animals, however, 

 and in some of the higher plants, complete separation is accom- 

 plished into male and female sexes each of which produces only one 

 kind of gamete. The sexes are also often conspicuously differ- 

 entiated in superficial character. These superficial differences may 

 be without evident connection with the essential sexual processes. 



It is evident that the one fundamental difference between the 

 sexes is the ability to produce the different kinds of germ cells, 

 yet when we consider that sex cells are, in their essential con- 

 stituents, the same except in connection with sex determination, 

 the difference seems entirely an adaptation to specific conditions 

 of reproduction. The demonstrated occurrence of sex reversal 

 in the domestic fowl and frogs further complicates the problem. 

 Since some birds and frogs have been both father and mother 

 during their lifetime, sex seems biologically much more a matter of 

 convenience than of necessity. 



The one great contribution of sexual reproduction for which 

 there is no known substitute is the reassortment of characters 

 with each generation through the combination of those present 

 in two individuals to produce a third. To conclude with this 

 statement is to confess our entire ignorance of the development 

 of sex. We can only point out the existing gradations in living 

 organisms and the one biological phenomenon which seems to 

 be an adequate reason for sexual reproduction. It remains for us 

 to consider the peculiarities of the germ cells and of the individuals 



