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THEORIES OF INHERITANCE AND DEVEIOPMENT 



mesoblast. After the primary differentiation of the germ-layers the 

 four entomeres do not divide again until a very late period (free- 

 swimming trochophore), and their substance always retains a charac- 

 teristic appearance, differing from that of the other blastomeres in 

 its pale non-granular character and in the presence of large oil-drops. 



Fig. 136. — Modification of cleavage by pressure in Nereis. 

 A. B. Normal 4- and 8-cell stages. C. Normal trochophore larva resulting, with four entoderm- 

 cells. D. Eight-cell stage arising from an egg flattened by pressure ; such eggs give rise to trocho- 

 phores with eight instead of four entoderm-cells. Numerals designate the successive cleavages. 



If unsegmented eggs be subjected to pressure, as in Driesch's echino- 

 derm experiments, they segment in a flat plate, all of the cleavages 

 being vertical. In this way are formed eight-celled plates in which all 

 of the cells contain oil-drops (Fig. 136, D). If they are now released 

 from the pressure, each of the cells divides in a plane approximately 

 horizontal, a smaller granular micromere being formed above, leaving 



