Causes of Tumors. 337 



isolation and transplantation of tissues is as yet not delicate 

 enough to prevent damaging the tissues and thus causing the 

 failure of the desired results. 



Cohnheim extended his theory of the embryonic origin of 

 timiors to a wider application by advancing the hypothesis that 

 at an early period of embryonal developinent more cells may be 

 formed than are needed in the construction of the organs and 

 that some of these cells may remain unused as "rests," retaining, 

 because of their embryonal nature, a marked capacity for prolif- 

 eration ; and that from such excesses of cellular material tumors 

 may develop. The views advanced by Cohnheim, to whom we 

 are indebted for first pointing out the important significance of 

 foetal isolation of cells in connection with tumor formation, are 

 coming to be recognized as valid ; that from isolated segmenta- 

 tion cells, which have actually been observed, tissues and organs 

 may originate, and in all probability, too, independent tissue 

 formations and therefore autoblastomata. 



In a somewhat different form practically the same hypothesis has been 

 recently advanced by J. Beard. Beard suggests that from every fertilized 

 ovum a number of germ cells originate (?), from which, however, only 

 one goes to form the embryo, the remaining wandering into the embryonic 

 mass and entering into the formation of the chorion (or the sexual cells?). 

 It may be possible that a tumor might develop from such wandering germ 

 cell if it stray to an improper part, and that such a tumor may be abso- 

 lutely nothing but an incompletely developed (rudimentary) embryo (which, 

 of course, is applicable for a number of tumors, especially the true 

 teratomata). 



As a further application of Cohnheim's theory, which it is 

 true afifords a satisfactory explanation, not for all but for a 

 number of tumors, Ribbert has suggested that cells may escape 

 from their normal connection also in extra-uterine life and thus 

 become misplaced and in consequence provide the nucleus for a tu- 

 mor formation. It has been repeatedly noticed that tumors have 

 developed after a traumatic lesion, at the site of injury by foreign 

 bodies, after contusion, etc., as osteomata of the jaw after contu- 

 sion, large chondromata in animals and man after fracture of the 

 ribs at the seat of the fracture, cancer of the cheek in man after 

 being hit by a stone, cancer of the lip at the place where the 

 mouthpiece of a tobacco pipe has been frequently rubbing. As 

 it is ver}' difficult to think of such cases being due to any specific 

 irritative influence of the foreign body which caused the injury, 

 we are forced to consider changes in the positions of the cells, 



