REPRODUCTIVE PROCESS IN ANIMALS 63 



substance become hard and stiff to form a framework 

 to support the soft protoplasm the skeleton. Masses 

 of cells become developed into contractile muscles for 

 pulling about the various parts of the skeleton, and 

 consequent movement of the body as a whole. Other 

 tracts of cells have to do with digesting and absorbing 

 the food : others with the getting rid of poisonous waste 

 materials. Others become developed into an elaborate 

 transport system the blood which distributes the food 

 material throughout the body and collects the waste 

 material ; still others into that marvellous nervous 

 system which has to do with receiving impressions from 

 the external world, with linking the various parts of the 

 body together and controlling their activities, and with 

 that wonderful process which we call thinking. 



The myriads of cells which constitute the adult body 

 become highly specialized for their various walks in life. 

 But this specialization brings in its train the loss of that 

 great primitive power the power of undergoing fusion 

 together syngamy with its accompanying drinking in of 

 that elixir of life which renews their vitality and enables 

 them to continue alive. And so it is that the body of 

 these higher creatures is doomed to suffer unavoidable 

 natural death. Whereas the living substance of the 

 Protozoon is potentially immortal provided that circum- 

 stances remain favourable and that from time to time 

 it unites in syngamy with other living substance, it may 

 go on living indefinitely the higher animal has on the 

 other hand its days upon the earth numbered, however 

 favourable may be the conditions under which it exists. 



But and here is one of the most fascinating features 

 of animal organization there lurk somewhere or other 

 in its body one or more clumps of cells which have not 



