214 LECTURES ON BIOLOGY 



The digestible parts are decomposed and assimilated, and the 

 refuse expelled in the same manner in which it was taken. 

 Thus the Amoeba floats along with an awkward movement, eats, 

 and grows. But the growth is not unlimited. When a certain 

 size has been reached the nucleus of the Amoeba begins to 

 constrict in the middle and changes from the globular form 

 into the shape of dumb-bells. Gradually the constriction 

 becomes more distinct and deeper, and finally we get two cell- 

 nuclei which wander to the two opposite cell-poles. Now the proto- 

 plasm of the cell-body also begins to draw itself out until finally 

 the two cell-halves of which each contains one of the two nuclei 

 remain connected only by a thin bridge of protoplasm (compare 

 fig. 51). In the end, this last connecting link breaks and instead 

 of the parent-Amoeba we have two daughter- Amoebae which are 

 exactly like the mother excepting in size. Like her they crawl 

 about in search of prey, quickly grow, and divide in their turn, 

 after some time, into two little Amoebae. How simple and yet 

 how wonderful ! This primitive process of reproduction, which 

 is not inaptly called a ' growth beyond the individual measure,' 

 is repeated through innumerable generations, for so far as our 

 experience goes there is no natural limit. In these lowest 

 organisms that which we call development is as yet absent, for 

 the young are at ' birth ' exactly like the mother, excepting in 

 volume. But growth without a change in the organism cannot 

 be described as development. Nor do these primitive organisms 

 know death. They possess, as Weismann expresses it, potential 

 immortality, because, as we have seen, the mother divides into 

 two daughter-individuals and continues to live in them. But the 

 Amoebae are not immortal for all that, for like other organisms 

 they can be destroyed by poison, heat, or serious mechanical dis- 

 turbances. Potential immortality only expresses the idea that 

 a physiological death the normal decay, which is a fundamental 

 necessity of the organism does not take place in the Amoeba 

 and many of its relatives among the protozoans. 



It is a difficult idea to grasp, for that death follows life and 

 must of necessity follow it is an idea so deeply rooted in the 

 human mind which is reminded of it daily and hourly that 

 it will not accept any exception of this law. The doctrine of 

 potential immortality of the unicellular organism was therefore 



