II. THE PERIOD OF ORGAN DEVELOPMENT 147 



phases: (1) an early phase, at the end of gastrulation, during 

 which a weak tendency to form these organs arises in the areas 

 concerned, and (2) a later phase, during which specific contact 

 inductors, e.g. retina, hemispheres of the brain, neural crest, 

 mouth endoderm, bring about the final determination of the 

 organs. The first phase, which we have called "predetermina- 

 tion", apparently takes place under the influence of the 

 archenteron roof, but it is probable that an even earlier 

 gradient-field in the ectoderm also plays a role (Nieuwkoop, 

 1947; Lombard, 1952). In any case, there are strong indications 

 that this predetermination of the organs of the head takes 

 place, in a common "head organisation-field", in the way dis- 

 cussed above (p. 125). This view is supported, for instance, by 

 the fact that treatment of the germ with lithium ions modifies 

 the pattern of the head as a whole. Probably only quantitative 

 differences in a "head evocator" play a role here. In the second 

 phase, the place and character of the organs would be irre- 

 vocably determined by qualitatively different contact inductions. 



The evocators probably originate as products of the cell 

 metabolism of the inductors, and diffuse into the environment. 

 If they are to exert an effect on the reaction system, the 

 cells of the latter must be sensitive to the action of just these 

 substances. This is often the case only during a very limited 

 period. Here, again, the chemical processes in the cells of 

 inductor and reaction system must be accurately adjusted to 

 each other for normal development to be possible. If the 

 evocator is not produced at the right moment, or if the reaction 

 system has not yet acquired its sensitivity by that time, or 

 has lost it again, development will be upset. Whether or not 

 these developmental processes will be interlinked in the correct 

 way, depends on the course of events in the previous phases 

 of development. This explains why a slight disturbance at a 

 certain moment may give rise to serious abnormalities in later 

 stages. 



The inductions cause the formation of the primordia of the 

 various organs. These primordia are groups of cells which all 

 possess the potency to form the organ in question. Initially, this 

 organ-forming potency is not equally strong at all points, but 



