148 C. M. CHILD 



certain respects at different levels of the body. The axial factor 

 appears, not only in the regulatory development of the different 

 organs of the single piece but in the regulation of pieces from differ- 

 ent levels of the body. 



In any single piece of Planaria which forms a new whole we can 

 see that under natural conditions the regulatory development 

 of the new posterior end — not merely the regeneration in the 

 stricter sense but the whole development — is a slower process 

 than the development of the head. Under conditions which 

 decrease metabolism this difference becomes beyond a certain 

 point not merely a difference in rate but a difference in capacity. 

 Instead of forming a tail more slowly than a head the piece now 

 forms no tail at all, but may still give rise to a head. A similar 

 relation exists between head-formation and pharynx-formation 

 and it has been shown above that the development of a posterior 

 end may be largely or wholly inhibited without entirely preventing 

 the formation of the pharynx. In every case the effect of the 

 depressing factor on regulatory morphogenesis in a piece from a 

 single zooid increases posteriorly along the axis. 



Moreover, the experimental data indicate as far as they go 

 that this axial factor is essentially quantitative rather than quali- 

 tative. The rate of the dynartiic processes or certain of them 

 evidently decreases from the anterior end posteriorly along the 

 axis and the apparent qualitative differences under experimental 

 conditions are due merely to the fact that under these conditions 

 the rate of reaction becomes so low that little or no morpholog- 

 ical effect is produced. In other words, the evidence thus far 

 points to the existence of an axial gradient in rate of reaction as 

 the fundamental feature rather than a gradation of substances, 

 such as Morgan and others have assumed to exist. It is prob- 

 able that such a gradient can and does produce secondarily a 

 localization of different substances or of different quantitative 

 relations in a complex of substances at different levels along the 

 axis. 



The evidence for the existence of an axial gradient in rate of 

 reaction presented in this paper is based on visible morphological 

 features. In the following paper another line of evidence will 



