Apbil 14, 1916] 



SCIENCE 



517 



inhibition differs with the level in the gradi- 

 ent. Similar differences in degree of in- 

 hibition along the axes of symmetry are 

 also present in such cases. In this way the 

 whole shape, proportions, and degree of de- 

 velopment of different parts can be modi- 

 fied and controlled to a high degree and in 

 two opposite directions. Similar results 

 have been obtained with other forms. More- 

 over, at least most cases of experimental tera- 

 togeny resulting from the action of chem- 

 ical agents and general environmental con- 

 ditions, as well as many teratological forms 

 observed in nature can be readily inter- 

 preted on this basis. Experimental tera- 

 togeny then affords, on the one hand, a 

 valuable check on other means for demon- 

 strating axial gradients, and, on the other, 

 finds a simple interpretation, at least as re- 

 gards many of its features, on the basis of 

 the general conception of metabolic gradi- 

 ents. 



Third, it is possible in some of the lower 

 animals to eliminate the original axes in 

 isolated pieces by means of narcotics, and 

 then to establish a new axis in a different 

 direction by subjecting the pieces to a gra- 

 dient in external conditions. The shorter 

 the piece, the less marked the original polar- 

 ity and the more readily new polarities 

 arise in response to the differential action 

 of external factors. Very short fractions 

 of the axis may be completely apolar in 

 their behavior. This fact indicates that 

 physiological polarity is not, as often as- 

 sumed, a property of the protoplasmic 

 molecule, but rather a function of proto- 

 plasmic mass, as it must be if it is funda- 

 mentally a metabolic gradient. 



Fourth, it has been possible to demon- 

 strate experimentally for certain of the 

 lower animals that a relation of dominance 

 and subordination exists along the major 

 axis. The apical region dominates all other 

 levels of lower metabolic rate within the 



range of its influence, and in the absence of 

 the apical region the highest level of the 

 gradient present dominates all levels below 

 it and within its range. The apical region 

 itself, however, is to a high degree inde- 

 pendent of other levels of the axis. In the 

 reconstitution of pieces it is capable of de- 

 veloping, at least to a very advanced stage, 

 and in the lower animals, apparently com- 

 pletely in the entire absence of other parts. 

 The development of hydranths or apical 

 portions of hydranths from short pieces of 

 Tubularia stem, which has been described 

 by various authors, is an example. Other 

 levels of the body, however, never arise in 

 reconstitution except in connection with 

 more apical or anterior levels, though an 

 apical end need not be present to determine 

 their formation. In axiate plants the rela- 

 tions are essentially identical as regards 

 the major axis. The dominance of the 

 apical region, the growing tip of plants, 

 over other levels has long been recognized 

 by botanists. Moreover, in plants the apical 

 region may arise in the absence of other 

 parts, and the development of other parts 

 takes place basipetally from it. 



In the experimental reproduction of vari- 

 ous simple animals, I have found that when 

 the metabolic rate in the apical region is 

 decreased, organs along the axis arise nearer 

 to the apical end and to each other, while 

 increase in metabolic rate of the apical 

 region determines their localization farther 

 away from it and from each other. If the 

 localization of these organs is determined 

 by a certain position in the axial gradient 

 this relation between localization and meta- 

 bolic rate in the apical region is easy to 

 understand, for when the rate is low the 

 gradient is shorter and the dynamic condi- 

 tions for a particular organ arise nearer 

 the apical end, while a high metabolic rate 

 means a longer metabolic gradient and the 



