METABOLIC GRADIENTS 



67 



tion of this fact. This *'law" is merely a statement 

 of the observed fact that in the development of the 

 animal from the egg organs first become morphologically 

 visible in that region which becomes the anterior or 

 apical end, and from this region morphogenesis pro- 

 ceeds posteriorly or basally in a regular, orderly manner. 

 In short, a gradient in morphogenesis exists along the 

 major axis of the body, the apical end preceding. In 

 addition to this major gradient more or less definite 

 morphogenic gradients appear in relation not only to 



Figs. 10, 11. — Two stages of cleavage of frog's egg, showing axial 

 gradient in cell size resulting from gradient in rate of division. 



the minor axes of the whole body, but also in relation to 

 the axes of particular organs or parts. In fact the law 

 of antero-posterior development is merely a statement 

 for the major axis of the more general law of axial 

 developmental gradients. 



Embryonic stages of a flatworm among the inverte- 

 brates and the chick among the vertebrates will serve 

 to show these developmental gradients. Fig. 12 is a 

 diagrammatic outline of the adult stage of a small 

 bilaterally symmetrical flatworm, showing ''brain," 



