38 



CELL-DIVISION AND GROWTH 



II. i 



the second is parallel to the first, or one blastomere may lie apart 

 from the other three. Should the eggs be now released from 

 the pressure, each blastomere becomes rounded off, and after 

 two more cleavages the sixteen-celled stage consists of two 

 plates of eight cells lying over one another. But if the pressure 

 is maintained, the spindles are horizontal and the blastomeres lie 

 all, or nearly all, in one plane (Fig. 22 c, d). 



FIG. 23. Segmentation of the egg of Echinus microtiibercitlatus 



under prcssuve. (After Ziegler, 1894.) 



a, 8 cells in one plane ; b, 16 cells, the last division having been 

 tangential ; c, d, 16-32 cells : the direction of the spindles in c is shown 

 by the line : it is in the greatest length of each cell ; e, 64 cells : a cross 

 signifies a vertical or oblique division, a line a horizontal division. 



i 



Ziegler has followed the segmentation of the compressed eggs a 

 step further (Fig. 23). As the figures show, the first two divisions 

 are at right angles to one another, while the furrows of the next 

 two phases are, roughly, parallel to the first and second. In the 

 next division sixteen to thirty-two cells the outer cells divide 

 radially, the inner more or less tangentially, these divisions 

 being, like the previous ones, at right angles to the compressing 

 plates. In the following phase, some cells (those marked with 

 a line) still divide in the same direction as before ; but in others 

 (distinguished by a cross) the spindle is perpendicular to the 



