VARIOUS KINDS OF CELLS: TISSUES 31 



which is practically nothing but a small dense nucleus and a 

 nbratile tail which is really a cilium or flagellum. When 

 the union of the two kinds of gametes, or, as it is termed, their 

 conjugation, takes place, the spermatozoon plunges into the 

 egg, breaking off its tail at the egg's surface. The head of the 

 spermatozoon, which, as we have seen, is a nucleus, then travels 

 towards the nucleus of the egg and unites with it. The com- 

 pound cell which results from the union of these two gametes 



termed a zygote, and the zygote, after a longer or shorter 

 Interval, proceeds to divide into a mass of cells termed the 

 embryo, which, through a variety of changes termed develop- 

 ment or ontogeny, become converted into an organism re- 

 sembling the parent. The two kinds of gamete are usually 

 produced by two different kinds of individuals ; that which 

 produces the eggs being termed the female, and that which 

 produces the spermatozoon being termed the male. In some 

 groups of animals, however notably in those that lead some- 

 what isolated lives and have little power of locomotion both 

 kinds of gametes are produced by the same individual. Such 

 an individual is termed an hermaphrodite. 



The difference between male and female is termed a dif- 

 ference of sex. 



Having surveyed the principal types of all, we must now 

 take up the consideration of the manner in which they are 

 combined to make up what are termed the tissues of the 

 animal. Out of the tissues in turn the organs of the 

 animal are constructed. The word organ is used to denote 

 a part of an animal's body which is adapted to perform a 

 particular office. Thus a man's leg is a locomotor organ, the 

 eye a visual organ, and so on. 



The word tissue is derived from a French word meaning a 

 piece of cloth, and indeed it is sometimes used in English in 

 that sense. In zoology it was first applied to the network 

 of cell and fibres which constitutes the basis of the skeleton. 

 This network is termed connective tissue, and the inter- 

 crossing fibres of which it is largely constituted do resemble 

 the threads which form the warp and woof of a piece of cloth. 

 Connective tissue surrounds every organ of the body and 

 separates the different organs from one another. In fact, as 

 Huxley has said, if we could imagine all the organs of an 



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