INTRODUCTORY 9 



ingested. The amoeba, therefore, has rudimentary senses, though 

 no sense organs, for it discriminates between objects with which 

 it comes in contact, and selects what it will swallow. You may 

 have to watch long if you wish to hit upon the moment of 

 capture and absorption, but diatoms are very frequently visible 

 in the body of an amoeba. There all the protoplasm they con- 

 tain is dissolved and assimilated, after which the flinty envelope is 

 got rid of. Proteids, and nothing but proteids, are- the amoeba's 

 food, and proteids are equivalent to protoplasm. Hence it 

 feeds on living organisms only ; inorganic matter is no nourish- 

 ment for it. The processes of eating, assimilating and ejecting 

 tha': I have described, it is able to carry on without any special 

 organ. But if an amoeba be examined carefully, a round trans- 

 parent space may often be seen in the greyish substance. This 

 / is what is called the contractile vacuole ; it contracts periodically 

 and suddenly disappears, then slowly reappears. The object of 

 this seems to be the excretion of waste matters in a fluid form, 

 though this work is performed also by the whole surface. The 

 contractile vacuole is the first step towards the formation of 

 special organs for the performance of the functions of life. 



It is well now to stain the amoeba with carmine or a very 

 weak solution of iodine. The animal will be killed by this, 

 and then there will be seen in its inner untransparent part a 

 roundish spot called the nucleus, which is the central citadel 

 of its life. When the cell structure of living organisms was 

 first discovered, the cell itself was thought to have no archi- 

 tecture of its own. It was a structureless unit. Then the 

 nucleus was discovered, and in many cases a nucleolus l within 

 the nucleus. Now the nucleus itself is found to have a compli- 

 cated organisation, and to be capable of undergoing changes, the 

 meaning of which can only be guessed. The description of these 

 I must put off for the present. 



We must now investigate the amoeba's method of reproduc- 

 tion. It multiplies by a process called fission. The cell divides, 

 the nucleus being in every case halved, and there are two amoebae 

 instead of one. But after a long series of fissions all the indi- 



1 This is not the same as the micro- nucleus mentioned below. 



