26 THE ORIGIN OF THE NERVOUS SYSTEM 



between different radii are in reality nothing more than 

 apico-basal or polar pattern, together with a surface- 

 interior pattern. In the more complex forms of 

 radial symmetry a number of radial axes or gradients 

 arise, and the radially arranged parts differentiate in 

 relation to these. In bilateral patterns there is not only 

 a gradient along the longitudinal or polar axis, but 

 bilateral gradients are also present. In at least most 

 bilateral invertebrates the high region of these bilateral 

 gradients is apparently represented by the median ven- 

 tral region and in the vertebrates by the median dorsal 

 region. The evidence indicates that the chief axial 

 gradients appear primarily in the superficial regions of 

 the cell or multicellular body. In many of the simpler 

 organisms, e.g., many Protozoa (Child, 19146) and plant 

 cells, they are present throughout life only or chiefly 

 in the superficial regions, but where definite localized 

 internal organs with an axiate pattern exist these also 

 show axial gradients in all cases examined, though these 

 organ gradients do not necessarily coincide in direction 

 with the primary gradients. As was pointed out nearly 

 five years ago (Child, 1915^, pp. 54, 60) the primary 

 gradient or gradients may persist throughout life, but 

 do not necessarily do so. They may undergo modifica- 

 tion and complication in various ways during the course 

 of development: the originally quantitative relations 

 may become qualitative, new gradients may arise in 

 certain parts or organs, in some cases pre-existing 

 gradients may be broken up, obliterated, or reversed, 

 one kind of axiate pattern may be replaced by another, 

 and so on. In all cases, however, the sequence of events 

 is definite and orderly, and many features of it can 



