294 ELEMENTS OF BIOLOGY 



fold the total heart tissue o£ an adult chick. There is no reason to 

 expect it to cease to grow; barring accidents of the environment 

 this tissue is immortal, so long as it does not differentiate and form 

 adult heart tissue. 



Differentiation, the process of specialization of tissues during de- 

 velopment, confers certain properties of structure and function, but 

 these are apparently antagonistic to growth and cell divisions. In 

 general the older the embryo the more sharply are the tissues de- 

 fined; and the higher the animal in the scale of complexity the more 

 completely does differentiation terminate growth and cell divisions. 

 For example, the nerve cells of the human body are frequently 

 regarded as one of the most highly organized types of cells. In- 

 crease in nerve cell number is said to cease during the first year 

 after birth and the brain reaches its maximum growth very early. 

 Once a human nerve cell body is destroyed, it is never replaced. 



Tumors and Cancers. The antagonism between growth proc- 

 esses and differentiation is also exemplified in abnormal growths in 

 the adult animal body. Such adventitious growths are commonly 

 called TUMORS and cancers; by scientists they are known as neo- 

 plasms. There are many types of cancerous tissue but in general 

 they form two classes, malignant and benign. Malignant cancers 

 consist of cells that carry on a continuous process of growth and 

 cell division, without differentiating into anything resembling nor- 

 mal tissue. Benign growths contain differentiated tissues such as 

 epithelium or fibrous tissue. Such tumors attain a maximum size 

 and often cease, or very nearly cease, to grow. Malignant neo- 

 plasms, on the other hand, increase incessantly. 



Regeneration. We have seen that highly differentiated tissues 

 are incapable of increase. In last analysis, this means that cells that 

 are highly specialized and differentiated have lost the ability to un- 

 dergo division and to produce other cells like themselves. When 

 such tissues are injured and parts removed, no replacement occurs 

 unless the new part or tissue is derived from some less specialized 



