292 PATTERNS AND PROBLEMS OF DEVELOPMENT 



or xenoplastic, or it may consist in isolation in water or some other 

 medium. Different degrees of fixity, stability, or irreversibility of deter- 

 mination are recognized according to the development of the part in dif- 

 ferent environments. If it shows capacity for self-differentiation in cer- 

 tain environments, not in others, determination is said to be "labile." 

 Often however, conclusions concerning fixity or irreversibility of deter- 

 mination are drawn from a single change of environment. Self-differentia- 

 tion in the altered environment shows, of course, that determination or 

 segregation (F. R. Lillie, 1927, 1929) has taken place, but that it is fixed 

 or irreversible in all environments does not necessarily follow. It has been 

 pointed out by Harrison (1933) and by Gilchrist (1933) that determina- 

 tion is or may be relative and may be evident in one environment and not 

 in another. Transplantation experiments with urodele fin ectoderm pro- 

 vide an excellent example of the relative character of determination 

 (Twitty, 1939). Many regions or parts found by experimental alteration 

 of environment to be more or less determined are not morphologically 

 distinguishable from other parts at the time of alteration, but their later 

 development shows that differences of some sort must have been present 

 at that time. 



The development of the concept of "formative substances" has led 

 many investigators to believe that determination in development results 

 from the presence in the part concerned of a substance or substances dif- 

 ferent from those in other parts. In the earlier stages of determination 

 formative effects of such substances may not yet have become sufficient 

 to be directly distinguishable. Development of morphological form con- 

 sists in local differences in growth rate, in cell movement, pressure, ten- 

 sion, turgor, viscosity, etc., and in the metabolic reactions of the proto- 

 plasmic system and the character of the substrate ; it is the expression of 

 an exceedingly complex action system. That the systems concerned in 

 determination of a hydroid tentacle, a planarian head, or an amphibian 

 limb differ from other parts of the individual — at least after a certain 

 stage of development — ^is evident; but the tentacle, the head, and the 

 limb result from a definite spatial activity pattern, not merely from pres- 

 ence of a particular substance. 



With the progress of experiment it becomes increasingly evident that 

 the term "formative substance" is a misnomer. We find formative pat- 

 terns with metabolism as an essential factor. The reaction patterns, rather 

 than substance or substances, are the formative agents. A particular sub- 

 stance may determine a certain kind of metabolism, but it is the spatial 



