THE REPRODUCTIVE CELLS. 55 



egg of a skate or of another gristly fish, are of course 

 formed after fertilisation. Egg-shells must be distinguished 

 from the egg-capsules or cocoons, e.g., of the earthworm, in 

 which several eggs are wrapped up together. 



The Male-Cell or Spermatozoon 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 two distinct parts : the essen- 

 tial " head," consisting mainly of nucleus, and the mobile 

 " tail " which is often fibrillated. A small median portion 

 connects head and tail. The spermatozoa of Threadworms 

 and Crustaceans are sluggish, and inclined to be amoeboid. 



It is not quite accurate to say that the spermatozoon is 

 homologous with the ovum, Both are true cells, and they 

 are complementary, but the spermatozoon has a longer 

 history behind it. The homologue of the ovum is the 

 mother-sperm-cell or spermatogonium. This segments as 

 the ovum does, but the units 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-cell divides and the various kinds of 

 segmentation in ova. In most cases the spermatogonium 

 divides into spermatocytes, which usually divide again into 

 spermatides or young spermatozoa. 



Maturation of the 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 this 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 tiny cells or polar 

 globules, the first containing half, the second necessarily a 

 quarter of the nuclear material which composed the germinal 

 vesicle. The remaining nucleus of the ovum is reduced to 

 a quarter of its original mass, but the number of chromatin 

 threads which it contains is the same throughout. It is 

 noteworthy that the second division follows close oi) 



