334 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



usually differing in certain respects from those noted in cancers, may be 

 found also in other types of abnormal growth, as, for instance, in certain 

 types of regenerative or of the so-called inflammatory growth, or even in 

 certain instances, of excessive hormonal-correlative growth. Cancerous tissue 

 behaves essentially like an originally normal tissue stimulated to grow and 

 to move under more and more abnormal conditions. 



However, essentially cancerous growth differs from other types of ab- 

 normal growth in that it is an irreversible process, whereas regenerative, 

 hormonal and inflammatory growth ceases whenever the causes which in- 

 duced these proliferations are removed. Once a normal cell has become can- 

 cerous, it may die, but as far as is known at the present time, it will not 

 return to the normal state. All kinds of tissues which have the ability to grow 

 may become cancerous; cancerous epithelial tissue is called carcinoma and 

 cancerous mesenchymatous tissue is called sarcoma. Also, embryonal tis- 

 sues may become cancerous and parthenogenetically developing eggs may 

 give origin to teratomas, in which many varieties of tissues may be repre- 

 sented. Cancerous growths developing from tissues of the adult organism 

 are classified in accordance with the character of the tissue from which they 

 are derived. In addition to the fully cancerous, malignant tissues, there exist 

 others which are in a transitional state. They form the so-called benign 

 tumors, in which the growth is increased and abnormal but slower than in 

 the typical cancers, and in which it takes place not by infiltration of the 

 neighboring tissues but by concentric extension, the differentiation of the 

 affected tissues usually occurring in a more normal manner than it does in 

 cancerous tissues. However, all kinds of gradations exist between normal 

 tissue, benign tumors and cancer, and while in the majority of cases a benign 

 tumor remains benign throughout the life of the individual, it may change 

 into a malignant one. When cancer particles are transplanted into other 

 animals of the same species, they may, in the new host, maintain their 

 malignant growth or they may become necrotic, and are then absorbed. The 

 readiness with which different tumors can be transplanted into different 

 hosts and individuals differs greatly. 



As to the factors which induce this transformation of normal into can- 

 cerous tissue, three main sets of conditions can be recognized : ( 1 ) a stimu- 

 lation of growth which usually extends over long periods of time and may 

 show various degrees of intensity, (2) genetic factors, and (3) viruses or 

 virus-like substances. 1. Stimulating factors: Hormones may function as 

 stimulators of the cancerous transformation and they elicit cancerous growth 

 in those tissues in which they induce, also, under normal conditions, growth 

 processes. Then there are special chemical, so-called carcinogenic substances, 

 tar and some substances which are constituents of tar or related compounds; 

 they are very efficient in causing cancer, but are not as selective in regard 

 to the tissues which they affect as are hormones. Dibenzanthracene, benz- 

 pyrene and methylcholanthrene are the best known among these substances. 

 Also, various injuries, inducing long-continued, regenerative growth, may 

 have similar effects. Ultraviolet light, X-rays and radium may in the end, 



