84 PATTERNS AND PROBLEMS OF DEVELOPMENT 



stitute to a single individual or retain their individuality and whether one 

 may dominate the other and alter its development. The question whether 

 fusion of individuals of different races or species is possible also arises. 



DISSOCIATION AND AGGREGATION OF CELLS 



Cells of certain sponges and hydroids can be dissociated and, in contact 

 with each other, may aggregate into masses, which may be varied in size 

 as desired. Under certain conditions these masses develop into complete 

 individuals. These cases raise questions of the origin of developmental 

 pattern and of the possibility of cell dedifferentiation and redifferentiation 

 in relation to the new pattern. 



GENERAL PURPOSES OF THESE METHODS 



These methods of experiment have been developed in the attempt to 

 obtain information concerning developmental potentialities and potencies. 

 By means of them we endeavor to discover whether, or to what extent, 

 realization of potentialities or developmental expression of potencies is in- 

 trinsic in the part concerned or is dependent either on a particular regional 

 relation to other parts of the organism or on general factors of intraorgan- 

 ismic environment, and whether or how it is affected by different nonor- 

 ganismic environments. Analysis of physiological dominance, both in 

 preventing physiological isolation and in determining the course of de- 

 velopment of subordinate parts (induction), has been greatly advanced by 

 these methods. 



When isolated or transplanted parts undergo reconstitution, that is, al- 

 teration of pattern, we infer that they or some of their cells are not so 

 stably differentiated or their course of development so unalterably estab- 

 lished that they cannot react to the altered conditions, and that their de- 

 velopmental or other behavior as parts of the intact individual must be de- 

 pendent, at least in part, on factors in their intraorganismic environ- 

 ment. If they continue to develop or otherwise behave as in the intact in- 

 dividual, intrinsic factors independent of other parts are supposedly con- 

 cerned. As regards development, these differences are commonly distin- 

 guished, following Roux (1885), as dependent or correlative differentiation 

 and independent or self -differentiation. Actually, however, the dift'erence 

 involves not only differentiation but the whole pattern of developmental 

 and other behavior. A part may be independent in certain respects, de- 

 pendent in others. Moreover, as Roux pointed out, a development which 

 is independent as regards relation to other parts is dependent on relations 



