DYNAMICS OF MORPHOGENESIS 75 



isolated as long as the gradient persists, but develop under the 

 dominance of more anterior parts and therefore produce a tail. 

 By eliminating or reversing the gradient we may alter these 

 localizations. 



The headless piece represents a condition intermediate between 

 the development of a head and of a tail at the anterior end. 

 When rate y is sufficiently high in relation to rate x, to decrease 

 the original gradient below a certain minimum in the anterior 

 region of the piece, head-formation is simply inhibited, because 

 neither x nor y dominates completely. But if rate y should 

 become sufficiently high, as compared with rate x, to eliminate 

 the old gradient and establish a new one sufficiently steep in the 

 opposite direction in the anterior region of the piece, the region 

 X will become a tail instead of a head. These points will be fur- 

 ther discussed at another time on the basis of further experimental 

 evidence. 



V. THE FUNDAMENTAL REACTION SYSTEM OF THE SPECIES 



Attention has already been briefly directed in other papers 

 (Child '13 b, '13 c) to certain consequences for the problems 

 of inheritance and development of the conception of the organism 

 developed in these studies. If the head-region is a 'self-differ- 

 entiating' system which arises in development independently 

 of other parts and if other parts arise only in correlation with a 

 head region or with a part which has already arisen in this way, 

 we are forced to the conclusion that a single fundamental reaction 

 system is the basis of both development and inheritance. The 

 apical region or head region, or in animals which develop a 

 morphologically differentiated nervous system, the cephalic 

 region of the nervous system which is the dominant part of the 

 head, is a closer approach than any other part of the organism 

 to a morphological expression of this fundamental reaction 

 system. In the lower animals, as well as in the plants, we see 

 as a matter of fact that an isolated cell or group of cells capable 

 of development produces an apical region or head except where 

 it is prevented from doing so by correlation with already exist- 

 ing apical regions or heads. 



