420 PATTERNS AND PROBLEMS OF DEVELOPMENT 



In most of the experiments described thus far in which the oxygen 

 differential is certainly or probably a factor in determining where a hy- 

 dranth or hydranths shall develop, it is primarily a regionally selective 

 factor. In the pieces with transverse cut ends it determines which of two 

 possible polarities shall develop; the activation following section and the 

 resulting dominance and gradient do not depend on the oxygen differen- 

 tial, but they are the factors directly concerned in determining the axiate 

 pattern of the hydranth that develops. The oxygen differential determines 

 that this dominance and gradient at one end shall be adequate for hy- 

 dranth development, and in determining the intensity of activation at the 

 end or surface of a piece it plays a part in determining the length of the 

 resulting gradient and the scale of organization of the hydranth, and to 

 that extent is concerned directly in determining the polar pattern. In the 

 aggregates of dissociated cells the contact-free-surface differential, prob- 

 ably chiefly or wholly an oxygen differential, may apparently be directly 

 concerned in determining the polar gradient. However, whether the dif- 

 ferential acts merely as a regional activator or directly as determiner, the 

 final result is determination of the polarity of the whole piece or aggregate. 



These experiments with hydroids have been discussed at some length 

 because they appear to be particularly significant for the problems of 

 developmental pattern and polarity. If a differential in oxygen tension 

 can determine a physiological polarity that can be distinguished as a 

 gradient and becomes the basis of an axiate pattern with definite differ- 

 entiations along its course, the most logical and obvious conclusion is 

 that physiological polarity in its simplest terms originates as a gradient 

 involving differences in rate of the basal metabolism characteristic of the 

 protoplasm concerned. 



DETERMINATION OF RECONSTITUTIONAL PATTERN BY OTHER 

 ENVIRONMENTAL FACTORS 



Light has been shown to be an important factor in determining axiate 

 pattern in many plants, but influences chiefly the course of vegetative 

 development. In regeneration of the alga Bryopsis, development of a 

 new thallus axis instead of rhizoid axis can be induced by light at the 

 proximal cut end, although the plant has a well-defined polarity (Noll, 

 1900; Winkler, 1900a). This is one of the few plants in which reconstitution 

 occurs directly from the region of injury, as it does very generally in ani- 

 mals. According to Loeb (1895) and Goldfarb (1906, 1910), light is neces- 

 sary for reconstitution, or for continued reconstitution of hydranths in 

 certain hydroids; but this effect is apparently on axiate patterns already 



