72 



THE CELL AND PROTOPLASM 



in a multicellular organism may be more 

 or less stably predetermined or dependent 

 on conditions outside themselves, either 

 within or external to the organism. Here 

 we are concerned only with dependence on 

 external factors. 



The determination of a new axiate pat- 

 tern and development of a new individual 

 in coelenterates, flatworms and annelids by 

 the cell activation following transverse sec- 

 tion of the body, or in some cases partial 

 section or a local lacerated wound, are 

 familiar to every student of reconstitution. 

 In all these cases the cells primarily acti- 

 vated by the injury resemble a region of 

 primary excitation: first, in that they are 

 activated ; second, in that they determine a 

 gradient in physiological condition extend- 

 ing over a greater or less distance. At 

 particular levels of this gradient certain 

 organs differentiate. The activated cells 

 which give rise to the apical region of the 

 hydranth in the hydroid, the head in the 

 planarian and annelid constitute an in- 

 ductor. The hydroids Corymorpha and 

 Tubularia are excellent examples of this 

 form of development. When the naked 

 cylindrical stem or hydrocaulus of Cory- 

 morpha is sectioned transversely, reduc- 

 tion of methylene blue or Janus green in 

 low oxygen indicates an intense activation 

 of cells adjoining the cut end, even after 

 the rapid closure of the wound, and a 

 gradient of decreasing rate of dye reduc- 

 tion more intense than that present in the 

 intact stem extends from this region over 

 a distance of one to several centimeters, 

 according to experimental conditions. The 

 new hydranth develops from the higher 

 levels of this gradient, the two sets of ten- 

 tacles appearing at certain levels within 

 the region of the hydranth primordium. 



The point of chief interest in the present 

 connection is that we can control and vary 

 the length of this gradient by external fac- 

 tors and so determine the scale of organi- 

 zation of the hydranth and the develop- 

 mental fates of cells at particular levels. 

 By means of inhibiting agents, cyanide, 

 various anesthetics, low oxygen, low tem- 

 perature, etc., it is possible to determine 



that the gradient following section shall be 

 short, the two sets of tentacles localized 

 nearer the end and nearer each other and 

 the resulting hydranth small. Without ex- 

 perimental inhibition, with high oxygen, 

 with elevation of temperature, the new 

 gradient is longer, its upper parts repre- 

 sent higher physiological levels and the 

 hydranth primordium is longer, with its 

 tentacles localized at greater distances from 

 the end and from each other; that is, the 

 hydranth is organized on a larger scale. 

 Here the differentiations in development 

 of cells at different levels of the stem are 

 altered and determined through their posi- 

 tions in the gradient by quantitative dif- 

 ferences in external environment. In plan- 

 arian reconstitution scale of organization 

 and the developmental fates of cells at par- 

 ticular levels of the piece can also be 

 altered experimentally by external factors 

 which decrease or increase the activation 

 of the cells adjoining the cut end of the 

 piece from which the head develops and 

 so alter the height and length of the in- 

 duced gradient. Scale of organization in 

 annelid reconstitution can probably be al- 

 tered in the same way, but the necessary 

 experiments are still to be performed. 



In various hydroid species the activation 

 at both ends of an isolated piece is often 

 sufficient to induce hydranth development. 

 By means of electric current, by difference 

 in oxygen content or of hydrogen-ion con- 

 centration at the two ends of a piece it is 

 possible to determine hydranth develop- 

 ment at one end or the other. 



In development of the hydroid under 

 natural conditions one end of the polar 

 axis, the high end of the polar gradient as 

 indicated by respiration, dye reduction, 

 and differential susceptibility, becomes the 

 hydranth; the other end gives rise to one 

 or more stolons or holdfasts, each of which 

 is a gradient with high end at the tip, but 

 of course a gradient of different kind from 

 that of the hydranth-stem axis. Some hy- 

 droid species removed from natural condi- 

 tions to standing water in the laboratory 

 lose their hydranths in a few hours and 

 develop stolons in place of them, even from 



