DOMINANCE IN RECONSTITUTION 349 



dient is then only a partial gradient, and partial axial development re- 

 sults. It is also evident that development along each axis is determined 

 progressively from its apical end, for development in the partial forms 

 includes as much of the hydranth from the apical end basipetally as scale 

 of organization and available length permit. In the naked Corymorpha 

 pieces and aggregates the gradients corresponding to the partial axes can 

 be rendered directly visible by differential dye reduction soon after section 

 and before there is any indication of hydranth morphogenesis (pp. 97-98). 

 Occasionally a new axis, more or less inhibited by the dominance of a 

 hydranth or by external conditions, determines development of only prox- 

 imal parts of a hydranth — for example, a circle of proximal tentacles 

 without manubrium, or a few proximal tentacles, or sometimes only a 

 single tentacle at the apex of the axis. Apparently, the high end of the 

 inhibited partial gradient in these forms is not high enough to determine 

 the more distal parts of the hydranth. In most cases, however, either the 

 apical part of the hydranth develops or hydranth development is com- 

 pletely inhibited. Similar changes in scale of organization appear in the 

 reconstitution of other hydroids, of the scyphozoan Haliclystus, of the 

 scyphistoma of Aurelia, and of actinians. 



SCALE OF ORGANIZATION IN PLANARIAN RECONSTITUTION 



A wide range in scale of organization appears in planarian reconstitu- 

 tion. It has long been known from work of many investigators that iso- 

 lated pieces of different length may reconstitute complete individuals of 

 different size. A long and a short piece may be of the same width, but a 

 smaller head regenerates on the short, than on the long, piece; and the 

 individual resulting gradually approaches normal proportions, though it 

 may not attain them unless it is fed after reconstitution. On the other 

 hand, pieces of the same length from different body-levels differ as re- 

 gards relative size or length of parts. Size of head and distance between 

 head and pharynx, that is, length of prepharyngeal region, decrease from 

 anterior to posterior levels of origin of piece, or in species with fission zone, 

 to this zone, and both increase again in the posterior zooid (pp. 44-46). 



Histological studies of planarian reconstitution have led to different 

 conclusions concerning the method of formation of the new tissue regen- 

 erating at anterior and posterior ends of a piece. Some have maintained 

 that both the new tissue and the changes in shape and proportions of the 

 piece result largely or wholly from migration of cells with little or no 

 proliferation and growth, while others have observed mitoses following 



