26 



PRINCIPLES OF EMBRYOLOGY 



much harm is done by such a usage if one stops to think what one is 

 doing. 



As development proceeds, a region or district of competence gradually 

 turns into an individuation field. One can show the character of the 

 changes that occur in a diagram such as that of Fig. 14. At an early 

 stage, any part of the district can be caused to develop into the organ by 





^'"^^ mdividuatiort 



\ fuU 



n. n .^. 



I ! ' 7 ' ' 



Figuhe 1.4 



The development of a district of competence into an individuation field. 



The diagrams show a longitudinal section through a region of a developing 

 embryo. At the earliest stage (i) the precursor (A) of a certain organ has not 

 yet reached the threshold anywhere; this is the stage of a 'district of com- 

 petence'. In (2) the concentration of the active substance A has reached the 

 threshold and an individuation field for the organ is beginning to arise. In 

 (3) the individuation field extends outside the area in which the organ will 

 actually appear, which is dotted. The peripheral parts of the individuation 

 field depress the level of .4. This is the stage which persists in lower forms in 

 which regeneration is possible. In (4) the individuation field has contracted 

 until it is confined to the area of the developing organ; meanwhile the 

 substance ^ has disappeared in the outer parts of the region, partly owing to 

 the suppressive action of the individuation field and partly as a simple result 

 of the progress of development. 



bringing the concentration of some activity A above the threshold; but 

 the ease of doing this will be distributed in a graded way. Once the 

 threshold has been reached, and the organ begins to be formed, its in- 

 dividuation field gradually extends till at its maximum it covers a rather 

 larger area than that out of which the organ is produced, hi the peripheral 

 regions, the effect of the field is to suppress any tendency for a second 

 organ to be formed. As the formation of the organ proceeds, the extent 



