30 ZOOLOGY 



of essentially the same character as the fibres of tendon, and 

 this power is called into activity in case of a wound when 

 the outflowing blood is exposed to the air. Then a tangled 

 mass of fibres is formed which is called a clot. 



The reproductive cells are those which are cast off from 

 the parent, and which by growth give rise to a new organism. 

 In Protozoa they are of very many kinds, but both amongst 

 Protozoa and amongst Metazoa they may be divided into two 

 categories, viz. those that must unite with one another in pairs 

 before they can develop ; and those which are able to develop 



FIG. 8. Reproductive cells: A, ovum; B, spermatozoon; 

 A, head of spermatozoon; n, nucleus of ovum; nl, 

 nucleolus of ovum ; t, tail of spermatozoon. 



directly without such preliminary union. The former variety 

 are termed gametes, from the Greek word game, which signi- 

 fies a marriage ; the latter are termed asexual spores. In the 

 overwhelming majority of the Metazoa only gametes are pro- 

 duced, and these gametes are invariably of two kinds, viz. 

 comparatively large motionless gametes termed eggs, and small 

 actively moving gametes termed spermatozoa. The egg or 

 ovum is usually a rounded cell, with a considerable amount of 

 reserve food material termed yolk stored up into its cylo- 

 plasm : it has a large nucleus which takes the unusual form 

 of a rounded bladder with a small solid sphere inside it termed 

 the nucleolus. The spermatozoon consists of a minute head 



