INDIVIDUATION AND REPRODUCTION 203 



for some temporary changes during growth, but when new zooids 

 arise in the posterior region of the body (see pp. 122-25) each 

 zooid develops its own axial gradient. In other cases, such as the 

 segmented worms, where the body increases in length for a time or 

 indefinitely by the addition of new segments arising from a growing 

 region just in front of the posterior end, the gradient appears in its 

 simple form during the early stages of development, but undergoes 

 some secondary changes in the posterior regions of the body as the 

 new segments are formed. 



Up to the present time axial gradients have been found in all 

 forms examined, which include among unicellular forms some ten 

 species of ciliate infusoria, and among multicellular forms hydra 

 and several species of hydroids and sea anemones, eight species of 

 turbellaria, the developmental stages of the sea-urchin and starfish 

 and of the polychete annehds Nereis and Chaetopterus, several 

 species of oHgochete annehds examined by Miss Hyman, the 

 developmental stages of two species of fishes, and the cleavage and 

 early larval stages of salamanders and frogs. The variety of forms 

 examined with positive results leaves no doubt that the axial 

 metabolic gradient occurs at least very widely among axiate 

 animals. 



Where definite axes of symmetry exist there are indications that 

 metabolic gradients are also present along these axes, and these 

 gradients show a definite and constant relation to the course of 

 development with reference to these axes. 



These metabolic gradients are of course merely the expression 

 of a general condition and may undergo more or less variation in 

 steepness, i.e., in the amount of change in rate of metabohsm from 

 level to level, or may even disappear temporarily, or in later Hfe 

 permanently. But the fact that in each species gradients exist 

 which are characteristic and constant within certain hmits, at 

 least during the earher stages of development, is of the greatest 

 significance. 



In addition to these results, obtained chiefly by means of the 

 susceptibility method, there are many other data of observation 

 and experiment which point unmistakably to the existence of 

 axial metabolic gradients as a characteristic feature of axiate 



