RECONSTITUTIONAL PATTERNS IN EXPERIMENT 433 



tion of a part removed but the development of a new axiate pattern, be- 

 ginning with the apical region or head, which develops from the high 

 region of the gradient determined by section and isolation. Reconstitu- 

 tion of hydranth or head at the distal or anterior end of a piece from any 

 body-level except one immediately adjoining the original hydranth or 

 head is just as truly a heteromorphosis as a hydranth or head developing 

 at the proximal or posterior end. The one is "out of place" as much as 

 the other. The parts normally present between such a developing hy- 

 dranth or head and the level of section where it develops are absent until 

 later, when reorganization is induced by the new dominance and gradient. 

 At either end of the piece or elsewhere the reconstituting hydranth or head 

 represents beginnings of a new axiate pattern. 



Development of hydranth or head on an isolated piece can occur only 

 with a certain degree of physiological isolation of the cells concerned from 

 other parts of the piece ; this isolation results from the activation of cells 

 following section and isolation; this, in turn, alters or obliterates the old 

 polar gradient pattern and determines a new one. It is no exaggeration 

 to say that development of a hydranth or head or, in general, determina- 

 tion of a new gradient in reconstitution occurs in spite of the rest of the 

 piece, that is, in spite of the pre-existing gradient and organization. The 

 old pattern is more or less completely made over. On the other hand, 

 development of a basal or posterior end is primarily development of a 

 more or less subordinate part, determined either by the dominance of 

 parts anterior or distal to it or by inhibiting external conditions. The 

 hydroid stolon is an axiate pattern with its own dominance and gradient, 

 but still in some degree under the dominance of the hydranth-stem axis 

 or directly determined an external inhibiting factor. 



New pattern can be determined not only by the activation following 

 section and isolation but by implants and by external differentials — 

 light, gravity, centrifugal force, electric current, temperature, an oxygen 

 differential, H-ion concentration — in many organisms, both plant and 

 animal, by more than one of these factors. Even change of shape, prob- 

 ably involving local or regional stretching of the cortex and consequent 

 alteration of its physiological condition, is effective in some eggs. But 

 however the new pattern is initiated, it is, of course, the specific constitu- 

 tion of the protoplasm in which it appears that determines its char- 

 acteristics as developmental pattern. Induction by a dominant region of 

 reconstitution in other parts is very generally characteristic of reconstitu- 

 tional development, but a new gradient may be directly determined by 



