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 originally 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 successively. 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 contact with cancer cells, this does not hold good generally; and as we shall see later the study of animal cancer has shown that such transformations may take place under certain conditions.
The careful microscopic studies of many pathologists (among American investigators we might cite among many others: Councilman, Mallory, Ewing, LeCount, Warthin, Wilson and MacCallum) have contributed 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 evidence 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
