DIFFERENTIAL GROWTH I57 



tive unit, exercises specific functions not exhibited by the individual 

 elements as such. To mention a familiar though crude example, the 

 differential exposure to environmental factors of cells in the core of a 

 given group, as contrasted with those near the surface, is a property of 

 the collective not owned by its members. 



Cell groups under the influence of such collective factors, but prior to 

 the appearance of a manifest response on their part, are usually referred 

 to as "determined." There follow material transformations of the kind 

 discussed in the preceding chapter, introducing novel relations among 

 parts. Stepwise, in assembly-line fashion, the diversity of activities in- 

 creases. As initial time differences become accentuated, some parts of 

 the germ soon outstrip others in their tempo of differentiation. From 

 then on, influences of the more advanced ("determined") upon the less 

 advanced ("undetermined") parts become increasingly prominent. 

 These influences find essentially two forms of expression : ( i ) influences 

 spreading within a primary cell continuum, and (2) influences exerted 

 from one cell continuum upon another contiguous one, usually after 

 secondary junction. 



Influences of this type are customarily referred to as "inductions." 

 The term is purely descriptive and covers a considerable variety of het- 

 erogeneous phenomena which have often no more in common than the 

 label. There is not the least justification for the supposition that a single 

 operative mechanism underlies them all, much less so a single chemical 

 entity. Much more pertinent than the illusory search for a master solu- 

 tion to the "induction" problem is an objective analysis of each and 

 every one of the various concrete instances of induction that have thus 

 far been recognized. Therefore, instead of discussing induction in 

 general, let us concentrate on two specific examples, one for each of the 

 categories mentioned above. 



The first example concerns what might be called "homoeo-induction," 

 that is, the progressive recruiting of cells for a given type of differentia- 

 tion, spreading from a focal area like an infectious wave (e.g. the an- 

 tero-posterior progression of myotome formation). One gets the im- 

 pression that cells which have attained a certain differentiated character 

 can communicate their state to their neighbors, which then pass it on 

 further, and so on down the line. The experimental test lies in trans- 

 planting a differentiated fragment amidst less far advanced cells (37). 

 In many embryonic operations of this type, completion of the fragmen- 

 tary graft structures by recruitment from surrounding cell sources has 

 been observed, but in no case has the precise mechanism been revealed. 



