SO-CALLED FORMATIVE SUBSTANCES. 177 



In considering the case of Tubularia Morgan ' makes the fol- 

 lowing statement which may perhaps serve to illustrate the nature 

 of his hypothesis : "We may assume that the gradation of the 

 material is of such a kind that the hydranth-forming material 

 decreases from the apical toward the aboral end. The formative 

 influence acting from the exposed end inward (the stimulus of the 

 water on the free end), finds a prompter response when it acts in 

 the direction of decreasing amounts of hydranth-forming material 

 (which has the same gradation as that in the hydranth itself) than 

 when acting in the reverse direction (namely at the aboral end). 

 Therefore the oral polyp, as a rule, develops first. For its de- 

 velopment it needs certain nutritive material. This it finds either 

 in the coenosarc or in the circulation, and uses the materials as it 

 develops. In consequence the cut surface at the basal end can- 

 not get the material necessary for it to develop into a hydranth 

 and it either remains undeveloped or produces a stolon." 



Besides the assumption of the gradation of material this state- 

 ment of the hypothesis contains a number of other assumptions. 

 The rapidity of response to the stimulus depends on the direction 

 of gradation of material ; a gradation of hydranth-forming sub- 

 stance exists in the hydranth itself. The hydranth takes up 

 nutritive material and so prevents the aboral end from obtaining 

 it, so that this produces a stolon or nothing. But why should 

 the rapidity of response depend on the direction of gradation ? 

 What reason is there for supposing a gradation of hydranth-form- 

 ing substance in the hydranth itself? Why should a stolon 

 develop because the aboral end cannot obtain the nutriment 

 which is taken by the developing hydranth ? Does the develop- 

 ing stolon require no nutritive material or does it require a dif- 

 ferent kind from the hydranth ? If the latter, then stolon -formation 

 and logically also hydranth-formation are due not to stolon- 

 forming and hydranth-forming substances laid down in the stems 

 in regular gradation but to nutritive substances occurring appa- 

 rently anywhere. Moreover, we are told that the formative 

 influence is the stimulus of the water on the free end. If this is 

 the " formative influence " what need is there for hydranth-forming 

 substance ? 



'_/<>;. Exp. Zool., I., 4, 1904, pp. 587-588. 



