82 



COLLEGE ZOOLOGY 



In one of these divisions the chromosomes, which united to form 

 the bivalent chromosomes, separate, one single or univalent 

 chromosome going to each daughter cell. This is the only known 

 case in cell division where entire chromosomes are separated 

 from one another, except the corresponding stage in oogenesis. 

 It is known as a reduction division because it results in a reduc- 

 tion in the number of chromosomes to one half in the daughter 

 cells. After these two maturation divisions, as they are called, 

 the spermatids are metamorphosed into spermatozoa (Fig. 46, B). 



PRIMORDIAL 

 GERM-CELL 



OOGONIA 



MULTIPLICATION 

 PERIOD 



» • • • | • 



f \ V. GROWTH 



PRIMARY / # - \ r ^RIOD 



OOCYTE 1 \ \ 



SECONDARY 

 OOCYTES 

 (OVARIAN EGG "' " 1 • • I VTV I MATURATION 



AND POLAR.BODY) V J T\ > PERIOD 



MATURE EGG 



AND 

 POLAR BODIES 



Fig. 48. — Diagram illustrating the stages of oogenesis. The primordial 

 germ-cell is represented as possessing four chromosomes. 



The spermatozoa of various animals are usually easily distin- 

 guished one from another, but are mostly constructed on the 

 same plan. They resemble an elongated tadpole (Fig. 46, B), 

 having a head filled almost entirely with nuclear material and 

 a long flagellum-like tail which is the organ of locomotion; a 

 middle piece joins these two. The spermatozoa are th e active 

 germ-cells. It is their function to seek out and fertilize the 

 larger stationary egg cells. Frequently they are only to~^o o~o 

 the size of the egg, and in the sea-urchin, Toxopneustes, their 

 bulk is- about 5 (/o the volume of the ovum. 



