THE FINAL ADJUSTMENTS 1 23 



have an eflfect on the rate at which the processes 

 proceed. 



We find evidence which suggests, though it does not 

 prove, that the genes act in a similar way in the 

 more important earUer periods of development. For 

 instance, we find that when an organ is induced by 

 an organizer, the details of its shape depend on the 

 tissue itself and not on the organizer. If we graft 

 frog's skin in the mouth region of a newt, it becomes 

 induced to form a mouth, and then it forms a frog's 

 mouth, which is totally unlike a newt's. This means 

 that the organizer determines that it shall be a 

 mouth but that the competence determines the 

 particular kind of mouth. The competence of the 

 tissue, then, is always limited by the characters of 

 the particular species it belongs to. Now these 

 specific characters are, we may suppose on analogy 

 with other animals, themselves fixed by the genes. 

 If this is so, but we cannot yet prove it, then the 

 processes which bring the tissue into a competent 

 state must be controlled by the genes. 



In all these reactions the genes are not acting 

 alone, but are co-operating with the living matter 

 outside the nucleus, or cytoplasm. We can only dis- 

 cover genes by breeding experiments in which we 

 cross different varieties of animals, so that we only 

 know how genes produce differences in development. 

 Probably the cytoplasm provides much of the funda- 

 mental mechanism by which development is brought 

 about, and the genes act as directing and controUing 

 agents. The cytoplasm, on this view, would be, as 



