106 THE CELL IN RELATION TO 



which the cell adapts itself to the performance of 

 the varied special conditions of ceU-life. 



In considering the problem of differentiation 

 we are therefore led to inquire in what manner 

 the nucleus may be conceived to operate in the 

 determination of specific modes of protoplasmic 

 change. De Vries and many of his followers 

 have supposed that control of the cell is effected 

 by an actual migration of organized gemmules or 

 pangens from nucleus to protoplasm. But do 

 we really need to employ the pangen symbolism 

 in the consideration of this question? It seems a 

 sufficient basis for our practical attack on the 

 problem to assume that the control of the cell- 

 activities is at bottom a chemical one and is 

 effected by soluble substances that may pass from 

 nucleus to protoplasm or from protoplasm to 

 nucleus. Certainly it is to such a view that very 

 many of the chemical and physiological studies 

 in this field are now unmistakably pointing. The 

 opinion is gaining ground that the control of de- 

 velopment is fundamentally analogous, perhaps 

 closely similar,' to the control of specific forms of 

 physiological action by soluble ferments or en- 

 zymes. Experiment has established the fact that 

 certain forms of development are thus controlled 

 by substances, the " hormones," that may be ex- 

 tracted from the cells that produce them, and 

 upon injection into the body call forth their char- 

 acteristic results. Such an effect, for instance, 

 is the development of the cock's comb in the hen 



