PHYSIOLOGICAL GRADIENTS. 177 



nucleus persists from one cell generation to another means merely 

 that the pattern once established is persistent or inherited, al- 

 though it is difficult to determine to what extent the persistence 

 of the surface-interior conditions is concerned in the persistence 

 of the pattern. 



Viewed from this standpoint, cell pattern originates in the 

 differential between surface and interior in general, and axiate 

 pattern in differential between different 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 fun- 

 damental protoplasmic activity, but as regards cell pattern we 

 have at present no means of determining whether the differential 

 was primarily quantitative, though various lines of evidence point 

 in that direction. 



The presence of an axiate pattern does not imply the disap- 

 pearance 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 un- 

 doubtedly 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. Formation of entoderm by delami- 

 nation in all cells of a blastula for example appears to be a case 

 in point, and in the protozoa definite structural differentiation is 

 limited to the ectoplasm. But in the ectoderm of multicellular 

 animals, for example, we find numerous evidences of surface- 

 interior pattern ranging from the basal muscular extensions of 

 ectoderm cells in Hydra 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, however, as in the case of 

 the axiate pattern, the surface-interior relation represents merely 

 the physiological conditions under which the potentialities of the 



