178 C. M. CHILD. 



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 specialization which develops. If an axiate pattern is also 

 present, 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 of a photoreceptor. The surface-interior 

 relation merely determines that the physiological conditions under 

 which the hereditary potentialities of a given protoplasm to pro- 

 duce a photoreceptor are realized are conditions which arise in 

 development at or near the surface of the developing organism. 



The Gradients in Relation to Excitation and Transmission. 

 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 deter- 

 mines a higher rate of activity in some region, we may expect, 

 since living protoplasm is irritable, and since increased activity in 

 one region serves to some extent to excite adjoining regions, that 

 transmission from the region of increased activity will occur. In 

 the absence of highly specialized conducting paths we find that 

 protoplasmic excitation apparently undergoes a decrement in in- 

 tensity or effectiveness with increasing distance from the point 

 of origin, so that an excitation gradient results. Discussion of 

 the evidence bearing on this point is impossible in this paper, 

 but many facts indicate that the physiological relation resulting 

 from the differential exposure of a cell or cell mass is primarily 

 a relation of excitation and transmission and that the resulting 

 gradient is essentially the fixation or establishment of an excita- 

 tion-transmission gradient in the protoplasm through the modi- 

 fication of the protoplasmic substratum by the persistence or 

 repetition of the differential exposure. From this viewpoint the 

 gradient represents the most primitive sort of excitation-trans- 

 mission relation and its effects upon protoplasm. 



In a series of recent papers, R. S. Lillie has advanced a 



