THE PHYSICAL BASIS OF HEREDITY. 171 



cells of the individual. The difference in size is due 

 simply to the concentration of the food yolk and most 

 of the cytoplasm in one of the cells, the other three de- 

 generating, being sacrificed to the production of an egg 

 cell with the largest possible supply of nutritive sub- 

 stance in it. 



Turning to the development of the sperm cell we find 

 an exactly parallel series of stages, the end results, how- 

 ever, differing much in size. The mature 

 The sperm cell. , . j- 1 • ^ 



spermatozoon is an exceedmgly mmute 



cell, consisting typically of a cylindrical or conical 

 " head " containing a nucleus, a short cytoplasmic 

 "middle piece," and a long vibratile "tail," an organ 

 of locomotion differentiated out of the cytoplasm of the 

 cell from which the spermatozoon is derived. The 

 stages of multiplication, growth, and maturation are 

 passed through in the development of the spermatozoon 

 in the same order as in the egg development, save that 

 the period of growth does not include the storage of 

 food yolk in the primary spermatocyte, and the two divis- 

 ions of the maturation stages are equal ones, resulting 

 in the production of four cells of the same size, each of 

 which develops into a complete spermatozoon. The 

 accompanying diagrams of Fig. 9, taken from Boveri, 

 illustrate clearly the homologies existing between the 

 life histories of the two sorts of germ cells. The earlier 

 stages of ovogonia and spermatogonia are indistinguish- 

 able from each other; later in the period of growth the 

 increase in the size of the ovocyte marks it off from the 

 minute spermatocyte, but this distinction is merely one 

 due to non-living food material, and in no wise affects 

 the fundamental identity of the two. In the maturation 

 period the number of chromosomes in the nuclei of both 

 egg and sperm is reduced one half — on the one hand, the 

 ripe egg cell and three rudimentary egg cells (the polar 



