CANCER RESEARCH 35 



of the thyroid may be hardly distinguishable from the normal tissue 

 from which they are derived. 



Microscopic studies of early stages of cancers showed furthermore 

 that in many cases the growth starts at one well-defined rather limited 

 area of the affected tissue; and that all the tumor masses developing 

 subsequently are derived from the relatively few cells which were orig- 

 inally seen to proliferate. In other cases, however, the tumor growth 

 originates at several neighboring places simultaneously. And in still 

 other cases a certain tissue may, over a wide area of the body and even 

 at distant places, give rise to cancer formation simultaneously or suc- 

 cessively. In such cases we may assume that some change predisposing 

 to the development of cancer has taken place in the affected tissue, and 

 that relatively slight external stimuli, as for instance an injury, effect 

 of light, are sufficient to call forth the actual cancerous proliferation. 



Usually the proliferating cancer cells do not infect neighboring cells 

 with which they may come in contact during their proliferation. While 

 as a rule the neighboring normal cells do not become cancerous in con- 

 tact with cancer cells, this does not hold good generally; and as we shall 

 see later the study of animal cancer has shown that such transforma- 

 tions may take place under certain conditions. 



The careful microscopic studies of many pathologists (among Ameri- 

 can investigators we might cite among many others : Councilman, Mal- 

 lory, Ewing, LeCount, Warthin, Wilson and MacCallum) have con- 

 tributed many important and interesting facts concerning the structure 

 of various tumors, their resemblance to and deviations from the structure 

 of normal organs from which they are derived and such studies formed 

 the basis of a more detailed classification of tumors (von Hansemann, 

 Adami). We have learned that each organ or tissue gives rise to specific 

 tumors which not only differ in structure and metabolism, but also in 

 their proliferative and metastasizing energy. It will, however, not be 

 necessary to discuss these differences more in detail on this occasion. 



Before leaving the problem of human cancer we will briefly consider 

 what part heredity and microorganisms play as the cause of tumors 

 in man. 



Heredity is undoubtedly a factor in those cancers which develop 

 occasionally in cases of xeroderma pigmentosum or from pigmented 

 moles. We know that here the conditions preceding cancer are hereditary 

 and therefore cancer itself is indirectly hereditary. We furthermore 

 know that a certain class of cancers originates on the basis of embryonic 

 malformations, and inasmuch as these are under certain — as yet not 

 well defined — conditions hereditary, we may assume that certain cancers 

 belonging to this class are also hereditary. There is indeed some evi- 

 dence which points to this conclusion. There is for instance a case 

 known in which both the mother and her one and one fourth years old 



