CELL-DIVISION AND GROWTH 391 



words, each single division of the mesoblasts and each double division 

 of the ectoblasts splits off the material for a single somite ! The 

 number of these divisions, and hence of the corresponding somites, 

 is a fixed inheritance of the species. 



The causes that determine the rhythm of division, and thus finally 

 establish the adult equilibrium, are but vaguely comprehended. The 

 ultimate causes must of course lie in the inherited constitution of the 

 organism, and are referable in the last analysis to the structure of 

 the germ-cells. Every division must, however, be the response of the 

 cell to a particular set of conditions or stimuli ; and it is through the 

 investigation of these stimuli that we may hope to penetrate farther 

 into the nature of development. The immediate, specific causes of 

 cell-division are still imperfectly known. In the adult, cells may be 

 stimulated to divide by the utmost variety of agencies by chemical 

 stimulus, as in the formation of galls, or in hyperplasia induced by 

 the injection of foreign substances into the blood ; by mechanical 

 pressure, as in the formation of calluses ; by injury, as in the healing 

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

 of more complex physiological and pathological conditions, by any 

 agency, 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 ultimate 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 gain- 

 ing ground that the immediate causes of division, whatever their 

 antecedents, are to be sought in this direction. .That such is the 

 case is indicated by nothing more clearly than the recent experiments 

 on the egg by R. Hertwig, Mead, Morgan, and Loeb already referred 

 to in part at pages 1 1 1 and 215. The egg-cell is, in most cases, stimu- 

 lated to divide by the entrance of the spermatozoon, but in partheno- 

 genesis exactly the same result is produced by an apparently quite 

 different cause. The experiments in question give, however, ground 

 for the conclusion that the common element in the two cases is a 

 chemical stimulus. In the eggs of Chcetopterus under normal condi- 

 tions the first polar mitosis pauses at the anaphase until the entrance 

 of the spermatozoon, when the mitotic activity is resumed and both 

 polar bodies are formed. Mead ('98) shows, however, that the same 

 effect may be produced without fertilization by placing the eggs for 

 a few minutes in a weak solution of potassium chloride. In like 

 manner R. Hertwig ('96) and Morgan ('99) show that unfertilized 



