SUMMARY 439 



The agencies which determine the position of the various axes 

 involved in the gradient-field system may be of very various nature ; 

 they may be factors in the maternal environment (ovarian con- 

 ditions), biological factors (point of sperm-entry), or external 

 physical factors (as in the determination of the polarity of the egg 

 of Fiicus). In any case, they are external to the ^gg. They may also 

 operate at Ytry different times relatively to fertilisation. 



A number of chemical processes are set going by fertilisation. 

 These will proceed differently in the quantitatively different en- 

 vironments provided in different parts of the gradient-field 

 system, until qualitative differences are set up. In most cases, 

 these differences are at first not visible, and are presumably of 

 chemical nature ; this step in differentiation is therefore spoken of 

 as chemo-differentiation. These chemical differences appear at 

 first to be reversible (e.g. labile determination of the presumptive 

 neural tube region in the Urodele before gastrulation) but after a 

 certain point to become irreversible. From this moment onwards, 

 the organism consists of a mosaic of chemo-differentiated regions, 

 each determined to give rise only to one or a limited number of 

 kinds of structure. These are what we have called partial fields. 



The attainment of the mosaic stage often takes place under the in- 

 fluence of a dominant region or organiser. This may determine the 

 extent and form of the whole gradient-field within which chemo-. 

 differentiation occurs, as in Planarian regeneration, or may interact 

 with a previously established gradient-field orientated in another 

 direction, as in amphibian organiser grafts. 



The organiser may exert its effects at a distance, as does the re- 

 generated head on a cut piece of a Planarian, or may supplement 

 such distance effects by more powerful contact effects, as happens 

 when the amphibian organiser comes to underlie a certain portion 

 of the animal hemisphere, and at once determines it irrevocably as 

 a nervous system. 



Modifications of the gradients by external agencies will entail 

 alterations in the structures produced. These alterations may con- 

 sist in changed proportions, or in the total absence of certain 

 regions (temperature-gradient experiments with frogs' eggs, 



