10 LOEB— TUMOR GROWTH AND TISSUE GROWTH. [January 17. 



heals, and if the animal becomes pregnant, it begins to grow in the 

 same way as the normal gland, but is no longer able to produce milk. 

 It responds, therefore, only to certain stimuli, but not to others. 



If we persist still further and transplant a malignant tumor, a 

 cancer, of the mammary gland, we find that it no longer responds 

 to the stimuli of pregnancy. Such tumors do not seem to assume 

 a more rapid growth, nor do they ever secrete milk. The metabo- 

 lism of tumors differs, however, only in a greater or less degree 

 from that of the corresponding normal tissues ; and the tumor tissue 

 can even still continue to secrete certain substances in a similar way 

 to the normal tissues. This has been observed, for instance, in the 

 case of the tumors of the liver and of the thyroid gland, which latter 

 provides a so-called internal secretion, without which widespread 

 changes would take place in our body. It seems, therefore, in the 

 case of the tumor tissues that there exists a parallelism between its 

 loss of function and its capability to respond to chemical stimuli in 

 the body that normally excite and regulate function and growth. 



These observations bring us also nearer to an understanding of 

 tissue growth and tumor growth in general. Just now we men- 

 tioned substances of various kinds circulating in the body that regu- 

 late the growth of normal tissues and of tumors ; but there probably 

 exist a number of such substances. How else could we explain the 

 fact that the majority of tumors may be successfully transplanted 

 into the organism in which the tumor had originated, but not into 

 other individuals of the same species? Evidently there must exist 

 some difference between the chemical composition of the blood and 

 lymph of each individual of one species ; and each tissue of one indi- 

 vidual is more or less adapted to its own body fluid. Furthermore, 

 we have seen that tissues do not grow in animals belonging to dif- 

 ferent species ; there must, therefore, exist substances regulating 

 growth, which are the same in the same species, but differ in dif- 

 ferent species. Sometimes, however, certain families of white mice 

 differ among one another to a higher degree than the white mice 

 differ from gray mice. 



Such substances, however, can merel}- regulate the growth of 

 normal tissue and of tumor tissue ; they are not able to transform 

 normal tissue into tumor tissue. How the latter transformation is 



