DOMINANCE IN RECOXSTITUTION 343 



the appendage of a segment representing a lower level of the polar gra- 

 dient of earlier stages results. Further evidence that degree of activation 

 following section is concerned is the increasing frequency of homeosis 

 with advancing age of the animal and its occurrence at low, but not at 

 high, temperatures in certain cases. MetaboUc rate and regenerative po- 

 tency decrease, in general, with advancing age in postembryonic or post- 

 larval stages and certainly decrease with decrease in temperature. 



According to Zalpeter (1927), however, regenerating legs of the crayfish 

 may show characteristics of either more anterior or more posterior legs." 

 The similarity of the crayfish legs suggests that gradient differences in the 

 leg segments are not great and that slight variations in activation follow- 

 ing section may be sufficient to determine development of a more anterior 

 or more posterior leg on a certain segment. ^Moreover, a secondar\- gra- 

 dient with high end posterior is probably present at certain developmental 

 stages of the crayfish; if so, more anterior legs probably represent lower 

 levels of this gradient, and their development from more posterior seg- 

 ments may also result from unfavorable or inhibiting conditions. 



After removal of uropods and caudal ganglion Herbst ''igiy) found no 

 regeneration of uropods in most cases; but in a few individuals uropods 

 were recognizable, even though the caudal ganglion was not reconstituted. 

 This is not a case of homeosis; but, like those cases, it suggests individual 

 differences in degree of activation at the cut surface. Occasionally it is 

 sufiicient to determine uropod development in absence of the gangUon. 

 Differences in physiological age or other differences in physiological con- 

 dition, variation in degree of injur>-, maimer of healing, etc., may account 

 for the var}'ing results. Supposedly, the cells concerned in homeotic re- 

 generation are determined as appendage cells, perhaps as appendage of 

 a more or less hmited number of segments; but they are evidently not 

 fixedly determined as cells of the appendage of a particular segment. 



DOMINANCE AND SCALE OF ORGANIZATION 



By "scale of organization" is meant the spatial order of magnitude of 

 the developmental pattern in relation to an axis. In reconstitution of hy- 

 droids, planarians. nemerteans, annelids, and various other forms scale 

 of organization may vary greatly wdth physiological condition of the origi- 

 nal individual, with body-level from which the part is isolated, and with 



" Przibram mentions cases of replacement of posterior by an anterior wing in insects and 

 some other cases of appendages characteristic of more anterior segments, but conditions of 

 origin of these are unknown. 



