294 PATTERNS AND PROBLEMS OF DEVELOPMENT 



Transplantation experiments with amphibian material have shown in 

 many cases more or less labiUty as regards region of the body formed 

 but complete or relatively high stability as regards species or group char- 

 acteristics of organs formed (pp. 457,499). On the other hand, certain 

 species and group characteristics of larval form in echinoids and asteroids 

 can be altered to an extreme degree and in different directions by differ- 

 ential inhibition, conditioning, and recovery, that is, by altering the gra- 

 dient pattern (chap. vi). 



Doubtless many cases of determination do represent increase in specifi- 

 city over the undetermined condition and may be regarded as the begin- 

 ings of differentiation, an "invisible differentiation" (Gilchrist, 1937a, b). 

 The view that determination is not a gradual development of specificity 

 but a relatively abrupt restriction of potency of a part, and that it is al- 

 ways dichotomous, has been advanced by F. R. Lillie (1927, 1929). So far 

 as determination represents attainment of a certain degree of specificity in 

 a certain region of a gradient pattern, its gradual origin and development 

 seem equally possible, and Lillie 's critical period may represent the thresh- 

 old of attainment of a certain degree of specificity permitting self-differen- 

 tiation. Lillie regards determination as an independent variable in devel- 

 opment; but if it is a resultant of gradient pattern, it is by no means in- 

 dependent. 



The concept of differentiation is perhaps the most indefinite and most 

 loosely applied of any concerned with development. In a morphological 

 sense a part of a cell, a cell, or a cell group is commonly regarded as dif- 

 ferentiated when it differs visibly in structure from an earlier "embry- 

 onic," supposedly undifferentiated condition; sometimes a change of shape 

 has been regarded as differentiation. On the other hand, certain cells, 

 although visibly different in appearance from embryonic cells, are often 

 assumed to be indifferent or undifferentiated because they give rise in 

 reconstitution to parts other than those which they formed in the original 

 individual. Moreover, as noted above, chemodifferentiation is often in- 

 ferred in parts capable of self-differentiation, though other evidence is 

 lacking. The more advanced stages of histological and organ differentia- 

 tion are directly and clearly distinguishable in the higher animals, and 

 the morphological differentiation is generally paralleled by specificity in 

 chemical constitution and in character and products of metaboHsm; but 

 exactly when or how a particular differentiation begins, we do not know. 

 Conclusions as to presence or absence of differentiation are often purely 

 matters of opinion determined by more general opinions concerning de- 



