THE BIOLOGY OF THE CELL SURFACE 



this definition, I examine (i) the structure of the sperma- 

 tozoon and of the egg when they normally come together, 

 (2) the changes in both subsequent to union and (3) their 

 behavior at the time of union. The present chapter 

 embraces the discussion of (i) and (2), the succeeding 

 chapter confines itself to (3). 



For the majority of multicellular animals, the coming 

 together of the male and female germ-cells or gametes, 

 spermatozoon and egg, guarantees the perpetuation of the 

 species. This coming together of the gametes is fertiliza- 

 tion in the widest sense, without which the spermatozoa of 

 all and the eggs of most animals die. In this meaning 

 fertilization marks the beginning of a life though of course 

 both gametes are alive; it is the beginning of a new indi- 

 vidual brought into being through the loss of individuality 

 of each of the two cells which are co-partners in the process. 

 If fertilization in all animals which exhibit it were to occur 

 in the same mode and at the same time, our attack on the 

 problem of fertilization would not be so difficult; but we 

 encounter many differences. Always in biology must we 

 reckon with differences and seek to determine to what 

 extent they are "incidentia" or "differentia." The com- 

 plexity and diversity of animal structure and behavior are 

 either "incidentia" and as such defy reduction to simple 

 terms or, as "differentia," they mask some one common 

 factor to which we can reduce them. That there is one 

 feature common to all fertilization-processes we shall see 

 after we have evaluated the differences in the structure of 

 the gametes as well as those in the time and the mode of 

 their union. 



The partners in the fertilization-process are as described 

 above markedly different from each other in their develop- 

 ment to the moment of their coming together. In one 

 respect only are they similar: their nuclei contain each one- 

 half the number of chromosomes characteristic of the adult 



150 



