24 



PRINCIPLES OF EMBRYOLOGY 



surface of an early embryo (Fig. 1.3). Suppose that this has the following 

 properties: (i) that it has an anterior-posterior polarity, and that some 

 substance B is present in a gradient with a high concentration at the 

 posterior, sinking to a low level anteriorly; (2) that the micro-structure 



Figure 1.3 



The development of an (imaginary) embryonic field. 



(i) represents a section through a flat expanse of tissue, into which oxygen 

 is diffusing from one side, while some substance is being formed at A (con- 

 centration indicated by dotted line) and diffusing outwards; (2) and (3) show 

 sections through which the field which is developing as explained in the text. 



of the tissue is such that diffusion is faster within the plane of the tissue 

 than it is vertically through the thickness; (3) that over the whole lower 

 surface of the tissue, there is some precursor substance a which can be 

 autocatalytically converted into an active substance A. Now imagine that 

 at some particular point in the region, this conversion begins to occur, 

 and A to be formed (this position may be determined by the normal 

 surroundings of the tissue, or by some experimental means). A will now 

 diffuse slowly upwards through the thickness of the tissue, and more 

 rapidly within the tissue away from its point of origin in all directions. 

 Let us further suppose that the activity o£ A consists in causing the tissue 

 containing it to become heaped together into a thicker layer, and also 

 that its formation is inhibited by oxygen diffusing into the tissue from the 

 outer surface. Then where A first starts to form, we shall have a heaping 

 up of tissue, leading to a thinning of the surrounding ring of material; 

 and in this surrounding ring oxygen will penetrate a greater relative 



