292 CELL-DIVISION AND DEVELOPMENT 



produced by a different cause. In the adult, cells may be stimulated 

 to divide by the utmost variety of agencies — by chemical stmiulus, 

 as in the formation of galls, or in hyperplasia induced by the m- 

 iection of foreign substances into the blood; by mechanical press- 

 ure, as in the formation of calluses; by injury, as in the healmg of 

 wounds and in the regeneration of lost parts ; and by a multitude of 

 more complex physiological and pathological conditions, — by any 

 ao-ency in short, that disturbs the normal equilibrium of the body. 

 In all these cases, however, it is difficult to determine the immediate 

 stimulus to division; for a long chain of causes and effects may 

 intervene between the primary disturbance and the ultmiate reaction 

 of the dividing cells. Thus there is reason to believe that the for- 

 mation of a callus is not directly caused by pressure or friction, but 

 through the determination of an increased blood-supply to the part 

 affected and a heightened nutrition of the cells. Cell-division is here 

 probably incited by local chemical changes; and the opinion is gaining 

 <-n-ound that the immediate causes of division, whatever their ante- 

 cedents, are to be sought in this direction. The most promising field 

 for their investigation seems to lie in the direction of cellular pathology 

 throuf^-h the study of tumours and other abnormal growths. The work 

 of Ziegler and Obolonsky indicates that the cells of the liver and 

 kidney may be directly incited to divide through the action of arsenic 

 and phosphorus; and several others have reached analogous results 

 in the case of other tissues and other poisons. The formation of 

 <^ralls seems to leave no doubt that extremely complex and charac- 

 t"eristic abnormal growths may result from specific chemical stimuh, 

 and some pathologists have held a similar view in regard to the origm 

 of abnormal growths in the animal body. 



Suggestive as these results are, they scarcely touch the ultimate 

 problem The unknown factor is that which determines and main- 

 tains the normal equilibrium. A very interesting suggestion is the 

 resistance theory of Thiersch and Boll, according to which each tissue 

 continues to grow up to the limit afforded by the resistance of neigh- 

 bouring tissues or organs. The removal or lessening of this resistance 

 throuo-h injury or disease causes a resumption of growth and division, 

 leading either to the regeneration of the lost parts or to the forma- 

 tion of abnormal growths. Thus the removal of a salamander s limb 

 would seem to remove a barrier to the proliferation and growth 

 of the remaining cells. These processes are therefore resumed, 

 and continue until the normal barrier is re-established by the re- 

 generation. To speak of such a "barrier" or "resistance" is, how- 

 ever to use a highly figurative phrase which is not to be construed 

 in a rude mechanical sense. There is no doubt that hypertrophy, 

 atrophy, or displacement of particular parts often leads to com- 



