MALE CELL OR SPERMATOZOON. 



57 



earthworm, in which several eggs are wrapped up to- 

 gether. 



Male cell or spermatozoon. — This is a much smaller 

 and usually a much more active cell than the ovum. In 

 its minute size, locomotor energy, and persistent vitality, it 

 resembles a flagellate monad, while the ovum is comparable 

 to an Amoeba or to one of the more encysted Protozoa. 



A spermatozoon has usually three distinct parts : the 

 essential "head," consisting mainly of nucleus, and the 

 mobile "tail," which is often fibrillated, and a small middle 

 portion between head and tail, which is said to be the bearer 



Fig. 25. — Forms of spermatozoa (not drawn to scale). 



1 and 2. Immature and mature spermatozoa of snail ; 3. of bird ; 

 4. of man (A., head; m., middle portion; t., tail); 5. of sala- 

 mander, with vibratile fringe (f.) ; 6. of Ascaris, slightly 

 amceboid with cap (c.) ; 7. of crayfish. 



of the centrosome. The spermatozoa of Thread-worms and 

 Crustaceans are sluggish, and inclined to be amceboid 

 (Fig. 25 (6, 7)). 



Both ova and spermatozoa are true cells, and they are com- 

 plementary, but the spermatozoon has a longer history behind 

 it (Fig. 27). The homologue of the ovum is the mother 

 sperm cell or spermatogonium. This segments much as the 

 ovum does, but the cells into which it divides have little 

 coherence. They go apart, and become spermatozoa. 

 There is a striking resemblance between the different ways 

 in which a mother sperm cell divides and the various kinds 

 of segmentation in a fertilised ovum. In most cases the 



