ORIGIN OF AXIATE PATTERN 63 



(Fig. 68, p. 238) to various complicated sensory struc- 

 tures and the early differentiation of the neural tube 

 in the higher animals, and the relation of such features 

 of pattern to exposure of the surface appears obvious. 

 Here as in the case of the axiate pattern, however, the 

 surface-mterior relation represents merely the physio- 

 logical conditions under which the potentialities of the 

 hereditary mechanism of the protoplasm are realized. 

 The fact that the embryo possesses a surface determines 

 certain relations in the protoplasm to this surface and 

 the specific constitution of the protoplasm determines 

 the kind of reaction which occurs, the sort of specializa- 

 tion which develops. If an axiate pattern is also pres- 

 ent, physiological conditions are provided for differences 

 in reaction in relation to the surface at different regions 

 or levels of the body. This of course does not mean 

 that exposure to a special external stimulus is necessary 

 for the development of a particular superficial organ. 

 Light, for example, is not necessary for the differentiation 

 in the individual of a photoreceptor or an eye. The 

 surface-interior relation merely determines that the 

 physiological conditions under which the hereditary 

 potentialities of a given protoplasm to produce a photo- 

 receptor are realized, are conditions which arise in 

 development at or near the surface of the developing 

 organism. 



The differential exposure of the cell or cell mass to 

 the action of environmental factors is only the first 

 step in the establishment of the gradient. Admitting 

 that this differential exposure determines a higher rate 

 of activity in some region, we may expect, since living 

 protoplasm is irritable and since increased activity in 



