138 AGE, GROWTH, AND DEATH 



alone. As development progresses, the simple con. 

 dition of the cells is gradually obliterated, but we find 

 another condition arising which we call the differ- 

 entiated one. Differentiation is a process which goes 

 on in the body as a whole, but of course it is also a 

 function of each individual cell. 



We can see something of the process of differen- 

 tiation if we study the unicellular organisms, those 

 creatures, each of which is complete in itself, although 

 it consists of but a single cell, not of countless mil- 

 lions of cells as we do. The picture, Fig. 48, which 

 I have chosen to throw upon the screen, is one which 

 I think may have an additional interest to you, for it 

 is a photograph from the living cell known as the 

 parasite producing dysentery. Its name is En- 

 tamceba histolytica. Fig. 48 A is drawn from a living 

 specimen, which had thrown out three short protuber- 

 ances (pseudopodia) and had swallowed some foreign 

 body, which shows as a rounded dark mass. As the 

 nucleus did not show in this specimen, Fig. 48 B has 

 been added, a drawing from an individual which had 

 been preserved and artificially stained, by which double 

 treatment, as you see, the nucleus has been rendered 

 conspicuous. 1 Of course in the living specimen the 

 nucleus was equally present although hidden by over- 

 lying granules. Our Amoeba is a unicellular parasitic 



1 It gives me pleasure to thank Dr. W. T. Councilman for the loan of this 

 specimen, obtained from the intestine of a fatal case of amoebic dysentery. It 

 is on Dr. Councilman's brilliant investigations that our knowledge of this disease 

 is based. 



