S90 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



potential differences or differences in surface tension in cells of different 

 gradient-levels may bring the cells back to something like the original 

 order. Moreover, polarity in embryonic development of various coelen- 

 terates is highly labile, and it is possible in this case that cells of higher 

 gradient-levels may be sufficiently dominant to redetermine gradient- 

 level in other cells "out of placei." In certain other medusae {Geryonia, 

 Liriopc) calcium-free sea water and violent pipetting are necessary to 



B 



Fig. 184, A-C. — Dislocation of blastomeres of medusa, Aegineta, and reconstitution of 

 spherical blastula (after Maas, 1901). 



bring about dislocation of blastomeres, and development is more or less 

 abnormal. The difficulty of dislocation is apparently due to physical prop- 

 erties of the blastomere surfaces, and these may also prevent rearrange- 

 ment, or blastomeres may be injured by the experimental procedure. 



When the sea-urchin egg is subjected to a certain degree of pressure, 

 cleavage planes are in the direction of pressure, and eight- and sixtcen-cell 

 stages are flat plates of blastomeres only one cell thick ; micromercs may 

 not appear at the sixteen-cell stage. If the membrane is present, these 

 plates become more or less rounded after pressure is removed, apparently 



