40 CELL DIVISION [CH. 



below, is compressed between horizontal plates, not only the 

 first and second, but also the third segmentation division 

 will be in a vertical plane, because the cytoplasm is ex- 

 tended horizontally by the pressure and the spindles will 

 therefore lie horizontally. If. however, the egg is compressed 

 vertically by being sucked into a tube of diameter narrower 



FIG. 5. Diagram of eggs segmenting normally and 



compressed in glass tubes. 

 a, b. Normal first segmentation division, 

 c, d. First division horizontal instead of vertical, in 

 consequence of pressure. (After PRZIBRAM.) 



than that of the egg, the second segmentation division will 

 be in a horizontal plane, instead of the normal vertical one, 

 in consequence of the vertical extension of the cytoplasm 

 which causes the spindles to be vertical. The very unequal 

 cell-division which takes place in the separation of the polar 

 bodies from the egg seems to have a different cause, and 

 will be considered in a later chapter. 



