THE SCIENCE OF EMBRYOLOGY 



27 



of the field usually contracts again until it is confined to the organ itself; 

 and in the meantime the competence of the outlying parts of the district 

 disappears, so that the possibility of another organ appearing lapses. In 

 some of the lower animals, however, in which regeneration is possible 

 throughout life, the field remains in an extended form, controlling the 



.y\ 



Figure 1.5 

 Behaviour of active fields. 



(i) If a field is cut in half, each portion will develop into a complete unit; 

 they may each retain their original polarity (above) or one may be the 

 mirror image of the other (below). (2) If two fields are brought together and 

 allowed to fuse, they form a single field (most easily if their polarity is the 

 same). (3) If a central {a) or peripheral (t) region is removed from a field, the 

 remainder will still form a complete unit (above), while the small isolates 

 may each also form complete fields (below). 



competence of the peripheral parts of the district and suppressing its 

 ability to produce supernumerary organs. 



When an individuation field is active, it shows many properties which 

 remind one of the behaviour of magnetic or other physical fields (Fig, 

 1.5). For instance, if a field is cut in two each half may reconstitute a com- 

 plete field, so that two whole organs are developed. These are often mirror 

 images of one another. On the other hand, if two fields are brought to- 

 gether and allowed to fuse, they may rearrange themselves into a single 



