68 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS 



firm, as in the 



Fig. 32. — Diagram 

 of a typical sper- 

 matozoon. 



//., The so-called 

 " head " ; A., acro- 

 some, which first 

 attaches itself to the 

 egg-cell in fertilisa- 

 tion; CHK., chromo- 

 somes, the vehicles 

 of many hereditary 

 factors, if not of all ; 

 Cy., the protoplasm 

 of the head ; M.P., 

 " middle piece " of 

 the spermatozoon, 

 including the ccntro- 

 some (C.) ; T., the 

 locomotor tail, with 

 an axial filament 

 {A.F.) running down 

 the middle. 



chitinoiis coat around the eggs of many 

 Insects, and in these cases we find a 

 minute aperture (micropyle), or several 

 of them, through which the sperma- 

 tozoon can enter. The hard calcareous 

 shells round the eggs of Birds and Tor- 

 toises, or the mermaid's purse enclosing 

 the egg of a skate, are of course formed 

 after fertilisation. Egg-shells must be dis- 

 tinguished from egg capsules or 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 dis- 

 tinct parts : the essential " head," con- 

 sisting mainly of nucleus, and the mobile 

 " tail," which is often fibrillated, and a 

 small middle portion between head and 

 tail, which is usually the bearer of the 

 centrosome. The spermatozoa of Thread- 

 worms and most Crustaceans are sluggish, 

 and inclined to be amoeboid (Fig. 33 



(6, 7))- 



Both ova and spermatozoa are true 



cells, and they are complementary, but 



the spermatozoon has a longer history 



behind it (Fig. 34). 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 



difl^erent ways in which a mother sperm 



