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 
spermatozoon is an exceedingly minute 
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 
The sperm cell. 
