ORIGIN OF THE INDIVIDUAL 



239 



With the specialization of the egg along lines which render 

 it non-motile, it has devolved upon the sperm to assume the 

 function of seeking out the egg for fertilization. It does this 



FIG. 129. Human egg cell, X 415, and sperm cell, X 2000. A, Egg just 

 removed from the ovary, surrounded by follicle cells of the ovary and a clear 

 membrane. The central part of the egg contains metaplasmic bodies and the 

 large nucleus. Superficially there is a clear ectoplasmic region. (After Wal- 

 deyer.) B, two views of the human sperm, c, centrosome; h, 'head' consisting 

 of the nucleus surrounded by a cytoplasmic envelope; m, ne, middle piece; t, 

 tail or flagellum. (After Retzius.) 



in most cases by active lashing of its flagellum. This necessi- 

 tates a fluid medium in which the sperm can swim, and such 

 is provided by the environment in which the organism lives 

 or, in the case of most higher animals, where fertilization 



