276 VETERINARY PATHOLOGY. 



two successive generations. Hereford cattle are more frequently 

 affected with ocular tumors than any other breed. Injuries in 

 which there is a destruction of the tissue surfaces predisposes 

 to tumor formation, and, in many instances, in the human, sub- 

 surface wounds are a causative factor in the production of 

 epithelial embryonic tumors. A few cases of epitheliomata have 

 been studied in the horse and ox that were secondary to injuries. 



Tlie exciting causes of tumor formation have n(jt l^een delin- 

 itely determined, and this accounts for their undetermined char- 

 acteristics. Many theories have been advanced. Cohnheim ad- 

 ^•anced the idea that tumors were developed from misplaced 

 embryonic cells (cell rests). This thecjry has received the sup- 

 port of many pathologists. It is in accord with the general 

 biologic law that every cell produces a cell like itself or "like 

 begets like." The supporters of this theory have not proved it. 

 but the opponents ha\e not disproved it. All biologists recog- 

 nize the comple.xitv <>f embryonic develo])ment and are aware 

 of the possibilitv I'f cells becoming entangled or misplaced dur- 

 ing the formation period. Experiments have demonstrated the 

 possibility of successfully transplanting tissue, both embryonic 

 and adult, into a foreign location in the body. Transplanted em- 

 bryonic tissue, however, does not remain as such, l)ut soon be- 

 comes mature tissue, whereas malignant tumor cells have no 

 tendency to become matured. Transplanted tissue cells do not 

 infiltrate the surrounding tissue nor form metastases, Ijut remain 

 as distinct islands of cells. 



The parasitic theor\' has been supported bv a large number 

 of scientists. Xo doubt malignant tumors resemljle infectious 

 diseases in that the\' extend b)- metastasis and produce similar 

 elTects. However, the uncertain transmission of tumors from 

 one animal to another is certainly antagonistic to the parasitic 

 theory. 



Bashford, superintendent of the Imperial Cancer Research 

 Fund, London, England, successfully transplanted GG per cent 

 of sporadic tumors. On the other hand, Ehrlich has successfully 

 transplanted onlv about 14 per cent of sp(jradic tumors. Xo one 

 has produced a satisfactory proof of the isolation of any causa- 

 tive parasite. Bv Sf)me authorities bacteria were thought to be 

 the active agent in tumor production, and bv others protozoa 

 were claimed as the causati\-e factor, and, more recentlv, many 

 radical observers have proclaimed that yeast were the cause. 

 Some have even claimed that the tumor cells are parasites. 



Chemic disturbances, particularly changed chcmic reaction in 

 a tissue, is responsible for tumor formation according to some. 



