6o 



REPRODUCTION AND IJFE HISTORY. 



cocoons, e.g. of the earthworm, in which several eggs are 

 wrapped up together. 



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. 28. Forms of spermatozoa (not drawn to scale). 



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

 4. of man (h., head ; vi., middle portion ; ?., tail) ; 5. of sala- 

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

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



of the centrosome. The spermatozoa of Thread-worms and 

 most Crustaceans are sluggish, and inclined to be amoeboid 

 (Fig. 28 (6, 7)). 



Both ova and spermatozoa are true cells, and they are 

 complementary, but the spermatozoon has a longer history 

 behind it (Fig. 30). The homologue of the ovum is the 

 mother sperm cell or spermatogonium. This segments as 

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

 little coherence. They go apart, and become spermatozoa. 

 There is often a 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 



