SECT. 3] AND ORGANISATION 605 



paper should be consulted for further details. Other criticisms of 

 the general theory of physiological gradients have been made by 

 Wilson; Kingsbury; Lund; Allen; and others, but they do not 

 affect the main conclusions which have been described. 



The significance of Huxley's remarks in 1924, which have already 

 been quoted, can now be better appreciated. At the time of chemo- 

 differentiation, the various irreversibly determined regions differ 

 from each other by the presence not only of qualitatively different 

 substances but also by the presence of varying concentrations of the 

 same substance, according to the conception of axial gradients. "If 

 it is asked", said Huxley, "how we can imagine the process as 

 originating, the answer must, I think, follow some such lines as these. 

 During gastrulation every portion of the embryo has a definite 

 relation to the system of axial gradients. The two main gradients 

 extend both on the surface and internally and together constitute 

 a three-dimensional system of gradient co-ordinates. Every portion 

 of the embryo, therefore, has its own rate of activity corresponding 

 to its position in the existing co-ordinate system, and its own charac- 

 teristic proportions of yolk, glycogen, cytoplasm, etc., depending on 

 the previous effects of the apicobasal gradient during the growth 

 of the egg. When the organiser in the dorsal lip exerts its admittedly 

 as yet unexplained though not unparalleled action of initiating 

 differentiation, every region of the embryo is in a different con- 

 dition from every other. The substrate is different from place to 

 place, the result, therefore, also differs." The position of a given 

 point in the embryo on the physiological three-dimensional graph 

 is thus of greater importance than the proportions of primary 

 materials which it contains, and, further, the relative velocities 

 of processes going on there are more important than the actual 

 amount of substances of different kinds that happen to be present 

 there. The substances which can be distinguished in the unsegmented 

 ovum of a vertebrate are thus merely raw materials, and the or- 

 ganising influences are to a large extent expressible in terms of gradients 

 of activity. The first of these is probably determined before the egg is 

 laid at all, the second arises from the action of agencies external to 

 the egg approximately at fertilisation. We are thus left with the 

 conception of parts of the embryo as pacemakers of growth and 

 differentiation relatively to the rest: though in what exactly their 

 influence consists we do not as yet know. "The first step in organisa- 



