174 PATTERNS AND PROBLEMS OF DEVELOPMENT 



and several other species similar transformations occur. Ethyl urethane, 

 cyanide, MgS02, LiCl, neutral red, and CO2 all give essentially similar 

 results. Pieces of Plumularia setacea (California) in standing water lose 

 the original hydranths and develop numerous stolons from lateral 

 branches and from both cut ends of the chief axis, and no, or very few, 

 hydranths. In flowing water hydranths remain and hydranth-stem axes 

 develop from one or both cut ends of the chief axis, no stolons or very few 

 appearing. Sertularella miurensis (Japan) in standing water loses hy- 

 dranths and develops lateral and terminal stolons. In hypotonic standing 

 sea water, 75 and 50 per cent, and in flowing normal sea water hydranths 

 remain alive, and few or no stolons develop until inanition is far advanced. 

 These transformations of hydranth-stem axes into stolon axes obviously 

 result from differential inhibition. The most susceptible parts, the hy- 

 dranths, die or are resorbed, and the inhibited axes develop stolons. With 

 differential conditioning or recovery the stolons may give rise to hy- 

 dranths. Each stolon represents a growth gradient, the tip growing and 

 remaining in good condition, at the expense of other parts, in the absence 

 of food or after separation from the stock. As the stolon increases in 

 length, separation from the parent stock occurs because the cells at the 

 low end of the stolon gradient serve as food for cells nearer the tip. The 

 tip continues to grow at the expense of more proximal levels until reduc- 

 tion of the coenosarc to very small size. In Figures 60 {C) and 61 the 

 graded shading of the stolons indicates the gradation from the tips, where 

 cells are in good condition and fill the perisarc completely, to levels where 

 only a slender coenosarcal strand of atrophying cells persists. Unshaded 

 parts of these figures indicate empty perisarc. The stolon axis is a growth 

 gradient, the hydranth-stem axis is a differentiation gradient; the stolon 

 axis develops under somewhat depressing or inhibiting conditions, the 

 hydranth-stem axis under conditions more nearly optimal. The inhibiting 

 factor may be external, as it undoubtedly is in the transformations of 

 apical regions into stolons in standing water and in slightly toxic solutions; 

 or it may be physiological, as in determination of stolon development at 

 proximal levels of an axis by dominance of the hydranth-region (see 

 pp. 314-15). In any case, stolon development evidently indicates a rela- 

 tively low level of certain metabolic reactions. Oxygen is probably often 

 insufficient to support hydranth metabohsm at the more basal levels of 

 hydroid systems as their own stems and branches become more numerous 

 or in consequence of overgrowth by other forms. Many facts indicate that 

 the oxygen content of sea water under natural conditions is often not far 



