igos.] LOEB— TUMOR GROWTH AND TISSUE GROWTH. 5 



however, an ovum being in this case responsible for the new forma- 

 tion ; but also in this case the experimentally new-formed decidua, 

 as we call this tissue, dies. 



The latter variety of growth resembles much more closely the 

 real tumor-growth than do the former ; but in this case also the cell- 

 proliferation, and even the life of the newly formed cells, cease, 

 when the cause for the proliferation has disappeared. The cause 

 for the development of an artificial decidua is probably two-fold : 

 in the first place, a general chemical condition exists in the body at 

 that period ; and, under these predisposing conditions, a local stim- 

 ulus suffices to produce the tumor-like growth. These new forma- 

 tions might be called transitory tumors, because they have a definite 

 life-cycle ; they grow for some time, and then they disappear. 



In real tumors we find a similar but still more marked cell- 

 proliferation ; and they do not have such a definite life-cycle. Real 

 tumors do not retrograde usually, and may even grow, more or less, 

 during the lifetime of the bearer. Furthermore, we do not know 

 the cause of their origin, as we do in the case of the transitory 

 tumor. They grow, and we do not know why. If such tumors 

 grow more rapidly, and especially if they grow deep into the sur- 

 rounding tissue, digesting it, if parts penetrate into the blood- or 

 lymph-vessels and are carried away to distant parts of the body, and 

 here start a new growth, a so-called metastasis, then we call the 

 tumor malignant, or a cancer. 



We distinguish different varieties of cancer, according to the 

 tissue or variety of cells from which these cancers originate. The 

 malignant tumors derived from epithelial surfaces or gland cells, 

 we call carcinomata and the malignant tumors derived from the 

 connective-tissue cells, which unite the functionally more highly 

 developed cells, we call sarcomata. But from whatever tissue these 

 malignant tumors are derived, their main characteristics are identical. 



During the second half of the last century, pathiDlogists studied 

 very carefully the microscopical character of the/idif¥erent tumors ; 

 and they determined quite accurately the genesis oof these tumors 

 from normal tissues. They observed how cells began to grow down 

 into the adjoining tissues in cancer ; they described the general 

 spreading out of the new formation, and the character of the sec- 



