PART IV. 

 THE ANIMAL AS PART OF THE SPECIES. 



A. Reproduction. 



So far the animal has been studied simply as an individual. 

 But it has also to be regarded as part of a species, as an entity 

 which has not only to lead its own life, but to transmit that 

 life to offspring from generation to generation and thus to 

 secure the survival of the species. 



When a unicellular organism reaches such a size that the 

 proportion of mass to surface begins to interfere with nutrition, 

 the metabolism undergoes alterations which lead to the division 

 of the cell (p. 28). 



In certain cases conjugation between two cells may occur 

 and this appears to precipitate the process of division. 



In multicellular organisms the gametic cell or cells early 

 throw off the somal cell or cells from which all the tissues of 

 the body are developed. In Ascaris this occurs at the very 

 first division. 



The purpose of somal development, of the development of 

 the body as a whole in all its marvellous complexity, is simply for 

 the nutrition and protection of the gametic cells. These latter 

 are eternal, going on from one generation to another and in 

 each building up a body. The body is mortal, perishing with 

 the death of the individual. 



In all higher forms of animals conjugation of two gametic 

 cells precedes division and development. In many inverte- 

 brates this is not always necessary and reproduction without 

 conj ugation — parthenogenesis — may occur. 



In vertebrates and in a large number of invertebrates two 

 sets of gametes are formed — one the ovum which is generally 



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