242 PHYTOHORMONES 



experimental attack, and is only a restatement of the ob- 

 servations in different terms. If, however, the term be 

 interpreted as a gradient in the concentration of active 

 substances it not only becomes experimentally analyzable 

 but is in close agreement with the situation in plants. 

 Plants have, in fact, a gradient of auxin concentration which 

 is set up either by the apical auxin formation or by the 

 polarity, the latter probably being the cause of the former. 

 Heretofore polarity was only detected by its effect on regen- 

 eration or organ formation, and as so many other factors 

 affect this process it was a doubtful criterion even for the 

 existence of polarity. But now, by measuring the rate and 

 capacity of polar auxin transport we can measure polarity 

 quantitatively and this undoubtedly will lead to a better 

 understanding of this remarkable property of living matter. 

 Apex-to-base polarity provides an example of the simplest 

 type of "field." 



E. Organization 



As we have seen, the factors operating in regeneration 

 are those operating in normal development, and this suggests 

 that the same principles also underlie development in the 

 embryo. The effects of auxin in the plant compare very 

 interestingly with embryonic development in the animal. 

 Organization in the animal embryo, which may be defined 

 as differentiation according to a definite pattern, is slightly 

 different from Sachs' concept of organization referred to in 

 VIII A. In the amphibian gastrula, any part of the ecto- 

 derm is apparently able to differentiate into a neural tube. 

 That only one is formed is ascribed to the ''organizer," 

 viz., the dorsal lip of the blastopore, which as Spemann and 

 his school have shown will, after invagination, induce the 

 formation of a neural tube in that part of the ectoderm 

 against which it comes to lie. Recently attempts have been 

 made to isolate chemically the active principle of the or- 

 ganizer (see the review of Weiss, 1935). As a result, certain 

 substances have been found to have the power of causing 



