EXCITATION AND TRANSMISSION 87 



only at the cell surface, but may be more or less con- 

 tinuous in the interior according to the phase relations. 

 There is no reason why excitation should not be trans- 

 mitted along these internal limiting surfaces, so far as 

 they are continuous, and in this way penetrate the 

 protoplasm to a greater or less depth. The internal 

 limiting surfaces are doubtless not identical with the 

 external, and conditions in them may be less favorable, 

 but there are no grounds for believing that internal 

 transmission is impossible. If it is possible, the axial 

 gradients may exist not only at the surface but to a 

 greater or less depth in protoplasm, according to con- 

 ditions, and may in some cases involve the whole cell, 

 as is apparently the case in some eggs. This suggestion 

 also bears upon the question of cell pattern and surface- 

 interior pattern in general (p. 60). I believe, in short, 

 that the interpretation of axiate pattern in terms of 

 excitation and transmission and the more or less per- 

 sistent developmental protoplasmic changes which 

 result from it is not only the simplest interpretation 

 possible on the basis of our present knowledge but the 

 interpretation which accounts most satisfactorily for all 

 the known facts. 



It is impossible to review the literature of the sub- 

 ject here, but it is of interest to note that many authors 

 have suggested that polarity is a gradient of some sort. 

 Morgan (1904, 1906, 1907), for example, has at various 

 times advanced the idea of polarity as a gradation or 

 stratification of materials. Boveri (1910) maintained 

 that the polarity of the Ascaris egg must be a gradation 

 of some sort, and numerous authors have described 

 gradations in size or form of parts, rate of development 



