THE PURPOSES OF BEHAVIOUR 131 



ovum from which it develops divides by mitosis and the daughter- 

 cells divide again and so on. The cells grow in size after they 

 divide and in preparation for the next division. The embryonic 

 cells are assembled into organ-anlagen and then they undergo 

 tissue-differentiation. This is growth with the acquirement of a 

 specific, bodily structure. 



Growth with differentiation. And even when the specific pattern 

 of bodily structure has been attained, and while the organism 

 continues to grow, there is usually differentiation, so that it passes 

 through its life-phases and comes to exhibit structure that 

 appreciably changes. 



Reproduction. This is essentially growth with dissociation. 

 There is a limiting magnitude to individual growth and when 

 this is attained the organism dissociates some part of its body. 

 It divides by mitosis (in the case of a Protist) or it buds, or it 

 spawns eggs or emits spermatozoa, etc. These dissociated bodily 

 parts then undergo gro\\1:h in the above senses. 



Reprodiictio7i with differentiation. Reproduction may be a 

 strictly repetitional process. The cells of a tissue-culture go on 

 dividing — apparently ad infinitutn and with retention of a specific 

 form — or some races of animals (the Brachiopod Liiigula, for 

 instance) reproduce so that the generic bodily form has been 

 retained for hundreds of millions of years. But in general there 

 is a slow change in bodily form as the accompaniment of continued 

 reproduction. This is the transformist process. 



46^. Self-Preservation is the urge to continued individual 

 existence. It is the strongest life-urge. It is limited by inevitable 

 somatic death, either catastrophic or senile death. It is qualified 

 by reproduction which, in a way, prevents individual bodily 

 death but puts a limit to individual acquirements. 



47. ON THE MANIFESTATIONS OF THE LIFE-URGES IN 



BEHAVIOUR 



Ways of behaving, that is general kinds of bodily motions, 

 may be associated loosely with the urges. 



47^. Assimilation and its Manifestations. In many 

 organisms the behaviour in assimilating environmental materials 

 is of the very slightest kind. In plants, root-hairs, tendrils, leaves, 

 the petals of flowers, etc., move tropistically (see Section 51). 



