170 



C. M. CHILD. 



region in consequence of which it deprives other regions of ma- 

 terial. Doubtless these conditions are largely chemical in 

 nature. 



The existence of such conditions in this and other similar cases 

 justifies the conclusion that the stimulus to cell-division and 

 growth is not identical with the presence of excess of nutritive 

 material. Admitting the existence of a stimulus to division inde- 

 pendent of the presence or absence of nutritive material we may 

 expect to find conditions different within the cell or nucleus, ac- 

 cording as the changes which occur in consequence of the stimu- 

 lus are or are not balanced by the intake of material. In case 

 they are balanced, a condition of equilibrium is more or less 

 perfectly maintained or else changes in both directions from the 

 condition of equilibrium alternate more or less rhythmically. 

 In consequence of the complex character of the cell and the more 

 or less central position of the nucleus, the occurrence of rhyth- 

 mical cyclical changes is to be expected rather than the mainte- 

 nance of a condition of absolute equilibrium. 



If on the other hand the demands of the nucleus are not met 

 by the intake of material the condition of equilibrium is not 

 attained, and the processes in the cell, so far as this point is con- 

 cerned, are not cyclical, but acyclical or orthodromic. 



Now the nuclear phenomena which occur in connection with 

 mitosis are very clearly cyclical ; the condensation of the chro- 

 matin and the disappearance of the nuclear membrane are followed 

 by an apparent reversal which terminates in the reconstitution of 

 a nucleus and the resolution and distribution of the chromatin. 



In amitosis, on the other hand, no such cyclical changes occur. 

 The nuclear structure remains the same throughout the whole 

 process. There is no disappearance or transformation, followed 

 by return to the original condition, of any part. Apparently the 

 process consists essentially in increase in size in consequence of 

 formation of new nuclear material, followed by separation into 

 physiologically independent parts. Nothing in the visible phe- 

 nomena indicates the occurrence of reversal in direction of the 

 processes involved. Division itself may be due either to physical 

 factors or to the establishment with increasing size of more or 

 less independent regions or centers of activity. The appearance 



