62 THE ORIGIN OF THE NERVOUS SYSTEM 



parts of the surface of the protoplasmic or cell mass 

 concerned. As regards the axiate pattern, the evidence 

 indicates that the differential is primarily quantitative 

 and involves differences in the rate or degree of funda- 

 mental protoplasmic activity, but as regards cell pattern 

 we have at present no means of determining whether 

 the differential was primarily quantitative, though vari- 

 ous lines of evidence point in that direction. 



THE RELATIONS OF SURFACE-INTERIOR AND 

 AXIATE PATTERN 



The presence of an axiate pattern does not neces- 

 sarily imply the disappearance of a general surface- 

 interior pattern, either in the cell or the multicellular 

 organism. All organisms show some kind of surface- 

 interior pattern, at least in the superficial regions of 

 the body, and all the facts indicate that in the final 

 analysis such pattern arises through exposure of the 

 surface. The passage of cells to the interior of the 

 embryo by gastrulation is of course a feature of axiate 

 pattern, but conditions in the interior are undoubtedly 

 factors in determining the further differentiation of 

 such cells into the organs of entoderm and mesoderm. 

 Only in some of the simpler animals does the general 

 surface-interior differentiation arise in situ. Forma- 

 tion of entoderm by delamination in all cells of a blastula, 

 for example, appears to be a case in point, and in the 

 Protozoa definite morphological differentiation occurs 

 primarily and chiefly in the ectoplasm. But in the 

 ectoderm of multicellular animals we find numerous 

 evidences of surface-interior pattern ranging from the 

 basal muscular extensions of ectoderm cells in Hydra 



