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 
