130 



CLEAVAGE AND DIFFERENTIATION 



Fig. 6 1 

 Cleavage of the sea-urchin egg. Column A, normal cleavage as far as the 

 i6-cell stage (eight mesomeres, four macromeres, four micromeres), serving as 

 time-scale (read from top to bottom) for the other columns. By treatment with 

 hypotonic sea-water or shaking, the formation of the mitotic spindles can be 

 delayed : the other columns show the effects of increasing retardation of spindle- 

 formation. Column B, the first two cleavage spindles latitudinal, the third vertical 

 but so delayed that it falls within the period of micromere-formation : result, 

 four micromeres at the 8-cell stage. Column C, first cleavage spindle latitudinal, 

 the second fall within the period during which the spindles are rotated into the 

 vertical position ; they have not quite achieved it here and are oblique ; the third 

 cleavage spindles, at right angles to the second, are also oblique : result, two meso- 

 meres and two micromeres at the 8-cell stage. Column D, the first cleavage spindle 

 latitudinal, the second vertical, the third similar to the fourth of normal cleavage, 

 i.e. latitudinal in animal, vertical in vegetative cells: result, four mesomeres, two 

 macromeres, two micromeres at 8-cell stage, Blastomeres isolated at the 2-cell 

 stage cleave according to this pattern. Column E, cleavage of blastomeres isolated 

 at the 4-cell stage or of eggs cut into meridional halves (in which the mitotic 

 apparatus is so delayed that the first cleavage spindle coincides with the third 

 of normal cleavage and is vertical) ; the second (like the fourth normal) cleavage 

 spindles are latitudinal in animal, vertical in vegetative cells. (From Horstadius, 

 Acta Zool. IX, 1928, slightly modified.) 



