FACTORS IN ONTOGENY 



265 



alike in that they all contain the same number of chromosomes, 

 i. e., one half the number in the somatic 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 degenerate, being sacrificed to the production of 

 an egg cell with the largest possible supply of nutritive sub- 

 stance in it. 



In the development of the sperm cell (Figs. 149, 150), we find 

 an exactly parallel series of stages, the end results, however, 

 differing much in size. 

 The mature sperma- 

 tozoon is an exceed- 

 ingly minute cell, con- 

 sisting typically of a 

 cylindrical or conical 

 ''^head" containing a 

 nucleus, a short cyto- 

 p 1 a s m i c "middle 

 piece,'' and a long 

 vibratile "tail/' an 

 organ of locomotion 

 differentiated out of 

 the cytoplasm of the 

 cell from which the 

 spermatozoon is de- 

 rived. 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 sperma- 

 tocyte, and the two divisions of the maturation stages are 

 equal ones, resulting in the production of four cells of the 

 same size, each of which develops into a comi)lete sperma- 

 tozoon. 



The accompanying diagrams of Fig. 151, taken from Boveri, 

 illustrate clearly the homologies existing between the life 

 histories of the two sorts of germ colls. The earlier stages of 

 ovogonia and spermatogonia are indistinguishable from each 

 other; later in the period of growth the increase in size of an 

 ovocyte marks it off from the minute spermatocyte, but this 

 distinction is merely one due to nonliving food material, and 



Fig. 150. — First (A-C) and second {D H) matura- 

 tion of the spermatocytes of Ascaris megalocephala. 

 (After Brauer.) 



