THE VITAL UNITS CALLED CELLS 1 99 



rare cases occur. If it is literally true that life once "evolved" 

 and that the process of creation has never been repeated 

 the most primitive cells now known to us or their descendants 

 must have persisted without evolutionary change for a very 

 long time. This would mean that they constitute a self- 

 perpetuating reservoir of Hving forms arising from others 

 hke them which is not replenished by the creation of new 

 forms from inanimate material. 



It is interesting to note that certain disease-provoking 

 agents exist which are not cellular in structure. Some 

 investigators believe them to be living, while others think 

 them to be dead. In this category are placed the iruses 

 (literally poisons) of chicken pox, rabies, common warts 

 and certain other infective diseases. They are too small to 

 be seen, yet hke living cells they are capable of unhmited 

 multiphcation, or more correctly, of increase in amount, 

 if each ultimate particle is not an individual unit susceptible 

 of division to form two others like it. The viruses have never 

 thus far been found to develop de novo, that is to say in 

 the absence of preexisting viruses. They can only increase 

 in intimate association with hving cells, from which they 

 may have arisen in the first place. To determine just what 

 they really are is one of the most captivating problems in 

 cytology. 



CELLULAR BASIS OF INHERITANCE 



But it is in cell multiphcation, and in the associated 

 phenomena of inheritance, that we have one of our greatest 

 riddles. It would seem a simple matter to ascertain why a 

 cell divides to form two others like it, but it is not so. Some 

 of the changes that occur we can observe though we cannot 

 begin to explain them. They are not always alike and there 

 is still some difference of opinion in regard to details but 

 the general process is represented very diagramatically in 

 Figure 4. 



A. A cell just before division is represented. In it the 

 distinctive nuclear material, chromatin, is illustrated dis- 

 tributed in the space within the nuclear membrane. Just 

 above the nucleus two granules may be seen, usually referred 



