3 2 ZOOLOGY 



covering of cytoplasm envelops the head and is drawn out into 

 the projection known as the tail (Fig. io,D). The middle piece 

 contains the centrosome of the male cell when this is present. 



49. Early History of Ova and Sperm. — We formerly 

 thought of ova and sperm as actually formed by the body of the 

 parent. We now know that these germ cells are set apart early 

 in the history of each individual, before the "parent" body is 

 developed. They of all the cells retain their primitive undiffer- 

 entiated nature. The body cells become different in appear- 

 ance and powers, — as nerve cells, muscle cells, bone cells, and 

 the like. The body does not produce the germ cells; it merely 

 houses and nourishes them during their development. The 

 body cells and the germ cells develop side by side, each influenc- 

 ing the others in their development. 



After the first putting aside of the primordial germ cells in 

 the early life of the individual, the body cells have a period of 

 rapid division and growth. During this time the germ cells are 

 relatively quiet. As the parent animal becomes mature the pri- 

 mordial germ cells enter upon a period of activity by which they 

 produce such ova and sperm as are described in the preceding 

 sections. This period of activity of the germ cells shows three 

 stages: (i) a period of increase of the primordial germ cells 

 {oogonia and spermatogonia) by division; (2) a period in which 

 the last descendents of these divisions enlarge (becoming pri- 

 mary oocytes and spermatocytes) ; and (3) a period of maturing 

 or perfecting these (into ova and spermatozoa) . These stages are 

 suggested in Fig. 11. 



50. Maturation of Ova and Sperm. — This third stage, the 

 maturation or ripening of the germ cells, shows us some of the 

 most remarkable happenings to be found in all biology. Both 

 ova and sperm when ripe contain, as a rule, just one-half as 

 many chromosomes in their nuclei as are found in the primordial 

 germ cells, or in the body cells, of the species to which they be- 

 long. Somewhere in the history of the egg and sperm there is 

 therefore a reduction division of chromatic material in the nu- 

 cleus. It does not always take place in just the same way, but 

 the following will illustrate the process. The student should 



