THE SCIENTIFIC INVESTIGATION OF CANCER 215
in the case of aniTnalfl rather suggest that in human cases too heredity may be an important cause.
There are other conditions which may under certain circum- stances act as internal factors. In the human being certain very peculiar tumors are found which by their general appearance recall the structure of embryonic tissues. By embryonic tissue we mean those structures which are intermediate between the ovum and the adult or- ganism. These tumors are therefore called embryomata and they are found especially within the generative glands. It has been recently observed that eggs within the generative glands may begin spontaneously to undergo a certain development — ^to develop without fertilization. According to our observations such a development may proceed even to the early formation of the central nervous system. Such structures are, however, usually destroyed after a period of temporary growth; but it seems that under certain conditions they can develop into ma- lignant tumors, malignant embryomata, or teratomata. The factors therefore which favor the fargoing parthenogenetic development of the ova must be considered as in some cases playing the part of internal factors leading to the production of cancer.
Developmental malformation is a third factor which may cause cancer. These tumors are frequently found at an early age, but they may become apparent only in later life. In eitiier case they originate during the period of embryonic development. We know that develop- mental errors are not infrequently found in man as well as in animals. Not rarely cells are misplaced, partly or entirely separated from the normal environment. In most cases such abnormally placed cells do not result in serious consequences, but, in some, as we have said, they become the starting point for malignant tumors. Pigmented moles may be considered as developmental errors and it is a well-known fact that under certain conditions favorable to cancerous growth they have become malignant tumors. Thus again indirectly may heredity play a part in the origin of cancer.
ExTEBNAL Factors
Let us now briefly consider the external factors to which we referred. Generally speaking any mechanical stimulus acting over a long period of time and leading to inflammatory conditions may produce cancer. Thus we know that long-continued burning of the sldn by carrying stoves, as the natives of Kashmir do, and long-continued action of Boentgen rays, extending over a nimiber of years without the available precautionary means, have led, in many cases, to the production of cancer. Chemical stimuli also may lead to cancer. It is well known that in certain trades cancer is more frequent because of the action of the chemicals used. Paraffin workers are not rarely affected by cancer
�� �
