6o 



REPRODUCTION AND LIFE HISTORY. 



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. 29). 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 



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



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

 4. of man (., head ; ;;/., middle portion ; /., tail) ; 5. of sala- 

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

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



spermatogonium divides into spermatocytes, which usually 

 divide again into spermatids or young spermatozoa. 



Maturation of ovum. When the egg-cell attains its 

 definite size or limit of growth, it bursts from the .ovary or 

 from its place of formation, and in favourable conditions 

 meets either within or outside the body with a spermatozoon 

 from another animal. Before the union between ovum and 

 spermatozoon is effected, generally indeed before it has 

 begun, the nucleus or germinal vesicle of the ovum moves 

 to the periphery and divides twice. This division results in 

 the formation and extrusion of two minute cells or polar 

 bodies, which come to nothing, though they may linger for 



