60 REPRODUCTION AND LIFE HISTORY. 
worms and most Crustaceans are sluggish, and inclined to 
be ameeboid (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 
75 
Fic. 28.—Forms of spermatozoa (not drawn to scale). 
z1and 2. Immature and mature spermatozoa of snail; 3. of bird; 
4. of man (4., head; 7., middle portion ; ¢., tail); 5. of sala- 
mander, with vibratile fringe (4); 6. of Ascaris, slightly 
ameeboid 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 
