DOMINANCE IN RECONSTITUTION 



345 







formation of this primordium and its decrease in length from distal to 

 proximal stem-levels were discussed in chapter ii. Physiological condition 

 of the individual from which the piece was taken is also 

 a factor in determining primordium length. Pieces from 

 animals in good condition, as indicated by size and ac- 

 tivity of the original hydranth, generally develop long- 

 er primordia at given stem-levels than pieces from ani- 

 mals with small degenerating hydranths or none. Pri- 

 mordium length can also be altered by inhibiting and 

 accelerating conditions. It is increased by increase in 

 temperature, up to a certain limit, and by slightly 

 hypotonic sea water and is decreased by inhibiting 

 agents, anesthetics, KCN (Child, 1931), decrease in 

 temperature, etc. It is also decreased by the domi- 

 nance of another hydranth within a certain distance 

 of it. Figure 115 shows the difference in length under 

 different conditions in stems of the same diameter and, 

 so far as could be determined, in similar physiological 

 condition preceding experiment. Similar differences in 

 primordium length appear in the same stem with dif- 

 ference in length of piece, stem-level, or with develop- 

 ment at opposite ends of a long piece. Similar dif- 

 ferences in scale of hydranth primordium appear in 

 Corymorpha; but since perisarc is absent and the 

 pieces change their shape by contraction and ex- 

 tension, measurements of primordium length are less 

 exact. 



In both these hydroids primordium determination 

 very often occurs from both cut ends of a piece, and 

 bipolar or multipolar forms result. In these the portion 

 of each axis developing is determined by the scale of 

 organization and the portion of the piece occupied by 

 the other axis or axes. Each axis is complete from its 

 apical end basipetally, as far as determination and 

 development extend. In short pieces which become 

 unipolar with hydranth at the distal end, the activation at the proximal end 

 following section may be sufficient to shorten the gradient and hydranth 

 primordium at the distal end, that is, to decrease scale of organization 

 there. Under conditions resulting in difference in scale, short pieces of 



B 



Fig. 115, A, B. — 

 Extreme differences 

 in length of hydranth 

 primordium in Tubu- 

 lar ia pieces. A, high 

 temperature or hy- 

 potonic sea water; B, 

 physiological or ex- 

 ternal inhibiting fac- 

 tors. 



