Growth Integration 109 



dients are definitely related to the axes of the individual or 

 its parts, and are evidently expressions of axial metabolic 

 gradients. While the existence of such gradients indicates 

 the existence of gradients in activity of some sort, the 

 various kinds of gradients are not all necessarily pres- 

 ent where metabolic gradients exist. In some cases the vis- 

 ible gradient may be a gradient in rate of growth or in 

 protoplasmic constitution ; in still others a gradient in 

 sequence of differentiation, etc., and sometimes metabolic 

 gradients exist without any structural indications of their 

 presence. At best these various kinds of gradients are 

 merely general indications of differences in metabolic rate 

 and undoubtedly in many cases the visible^ differences along 

 an axis represent something more than differences in meta- 

 bolic rate. The important point is that visible indications 

 of graded differences in metabolic rate occur so generally 

 in definite relations to the chief axes of the body. 5 



One phase of this general statement is the developmental 

 correlation that various regions of the body in many lower 

 animals have with the head or anterior end, these regions 

 being developmentally dominated, in Child's expression, by 

 the anterior end proportionally to the distance of the re- 

 gion from the end. 



A typical case is furnished by flat-worms of the genus 

 Planaria, animals especially favorable for experiments in 

 regeneration, since they are very hardy to laboratory con- 

 ditions and have great powers of reconstituting themselves 

 from pieces of various sizes, shapes and positions cut from 

 them. "Any piece of the planarian body," says Child, "is 

 capable of giving rise to all parts posterior to its own level, 

 whether a head is present or not, but no piece is capable of 

 producing any part characteristic of more anterior levels 

 than itself, unless a head begins to form first." 6 



From a great mass of experimental evidence produced b^ 

 Child and others we have the following: "These facts force 



