THE AXIAL GRADIENTS IN HYDROZOA. 



gradually lose material to the higher levels and finally the coeno- 

 sarc of these levels disappears completely and separation of the 

 stolon occurs. In Fig. 10 the two chief stolons have already sepa- 

 rated from the original stock, and some of their longer branches 

 are approaching separation from each other. In Fig. 14 also the 

 two largest stolons are separated and a third is approaching 

 separation. In Fig. 15 the terminal stolon of Obelia is almost 

 separated, in Fig. 16 separation is complete, except as regards 

 perisarc, and in Fig. 17 an earlier stage is shown. 



After separation growth goes on as long as the regions of higher 

 rate are able to take material from those of lower rate. In such 

 stolons, even after a week or two of growth, the tip appears well 

 fed and the cell layers are thick while toward the base the layers 

 become progressively thinner and the cells more shrunken. 



It is not yet known whether the rate of oxidation increases 

 in advanced starvation in hydroids as it does in Planar ia and 

 various other animals, but apparently either this occurs at the 

 lower levels more rapidly then at the upper levels of the separated 

 stolon, or else the rate of oxidation in the upper levels decreases 

 as the supply of nutritive material decreases. Either change 

 leads gradually to the obliteration of the gradient, and as the 

 cells become more and more alike in condition, growth becomes 

 slower and slower and finally ceases. 



According to this interpretation then the continued growth of 

 such separated stolons in the absence of food from without is a 

 simple physiological consequence of the fact that they represent 

 physiological gradients and likewise the difference in appearance 

 of the ccenosarc from the well filled tip to the shrunken, almost 

 transparent base is another expression of the gradient. It may 

 be suggested further that such stolons fail to develop hydranths 

 in the more advanced stages of starvation because the gradient 

 cannot attain the steepness necessary for hydranth formation. 

 So far as I know, no other adequate physiological interpretation 

 of these various facts has been advanced. The separation and 

 continued growth of stolons may be an adaptation for purposes 

 of reproduction under unfavorable conditions, but even if this 

 is the case, the necessity for physiological interpretation still 

 exists. 



