96 HOW ANIMALS DEVELOP 



neural plate shall be induced by each part of the 

 organization centre. It therefore determines where 

 the secondary organizers like the eye-cup shall 

 arise, and then their individuation fields determine 

 where the tertiary organizers appear, and so en. 

 We cannot yet say how the individuation field gets 

 its effect, but we can describe what the effect is. 

 We find that the field is a region where there is a 

 tendency to build up one complete embryo, and 

 that if we add any extra material or take any away, 

 the material which is left will do its best, so to speak, 

 to turn into one embryo and not one and a bit. The 

 state of being one embryo seems to be a condition of 

 equilibrium to which the tissue tends to return. 



Perhaps a mechanical analogy will make this 

 clearer without being, at the same time, too mis- 

 leading. Think of a big railway sorting yard, like 

 the one in Fig. 25. You are looking down an incline 

 called the Hump. The wagons are pushed over the 

 Hump and go running downhill and are sorted out 

 by the systems of points into the various sidings. 

 Now an embryo is in some ways analogous to a set 

 of trucks sliding down the Hump. The first point, 

 which you see just in front of the nearest two trucks 

 in the picture, is the primary organization centre 

 and shunts off one set of trucks to the left, to become 

 skin, and another set to the right to become neural 

 plate. The next set of points are the secondary 

 organizers, which we discussed in the last chapter, 

 and they again sort out the neural plate trucks into 

 brain trucks and spinal column trucks, and the 



