io 4 



AN INTRODUCTION TO ZOOLOGY 



divide, each forming two spermatids. Each spermatid receives 

 one half of each chromosome from the secondary spermatocyte 

 that gives rise to it. The spermatids are then metamorphosed 

 into spermatozoa. 



The spermatozoa of various animals are usually easily dis- 

 tinguished one from another, but are mostly constructed on the 



PRIMORDIAL 

 GERM-CELL 



ULTIPLICATION 

 PERIOD 



PRIMARY 

 OOCYTE 



SECONDARY 



OOCYTES 

 (OVARIAN EGG 

 AND POLAR.BODY) 



MATURE EGG 



AND 



POLAR BODIES 



FIG. 50. Diagram illustrating the stages of oogenesis. The primordial 

 germ cell is represented as possessing four chromosomes. 



same plan. They resemble an elongated tadpole, having a head 

 filled almost entirely with nuclear material and a long flagellum 

 like tail, which is the organ of locomotion; the middle piece 

 joining these two is the centrosome. The spermatozoa are 

 the active germ cells; it is their duty to seek out and fertilize 

 the larger stationary egg cells. Frequently they are only 

 nr^Tnnr the size of the egg, and in the sea urchin, Toxopneustes, 

 their bulk is about -^-frWo the volume of the ovum (99). 



Oogenesis. The origin of the egg is called oogenesis (Fig. 50). 

 Stages are passed through by the germ cells corresponding al- 

 most exactly to those described under spermatogenesis (Fig. 49). 

 Before the growth period the germ cells which will produce eggs 

 are known as oogonia (Fig. 50; Fig, 51, a). At the completion 



