48 TEXT-BOOK OF EMBRYOLOGY. 



2. That each division plane tends to intersect the preceding division plane at 

 right angles. 



The first of these laws is apparently dependent upon the fact that the 

 nucleus tends to occupy the center of the protoplasmic mass which is to undergo 

 division. In the case of a spherical cell the spindle may lie in any diameter. 

 In case one axis of the cell is longer than the other axis, the axis of the spindle 

 corresponds to the long diameter of the cell. When the cell divides the division 

 plane always "bisects the spindle perpendicularly to its long axis. 



Applying these laws to cleavage in a spherical holoblastic egg, the first 

 division plane may be in any direction but must bisect the mitotic figure at 

 right angles to the long axis of its spindle. The result is two cells, the long 

 axes of which are parallel to the first division plane. The axes of the mitotic 

 spindles in these two daughter cells, coinciding with the long diameters of the 

 cells, are therefore at right angles to the mitotic spindle of the mother cell, and 

 the second division plane is therefore at right angles to the first. The result of 

 the second division is four cells and as the first two division planes were vertical, 

 the long axes of all four of these cells are vertical, as are also their mitotic 

 spindles. The third division plane to be at right angles to these spindles must 

 bisect them in a horizontal plane. The third cleavage plane is therefore 

 horizontal. 



Such regular cleavage as that seen in Synapta (p. 43) in which after the 

 first vertical cleavage, vertical and horizontal cleavage planes alternate with 

 perfect regularity and the number of cells is exactly doubled at each cleavage 

 through the ninth cleavage or 512 cells, is extremely rare. Thus in many forms 

 the regular doubling of the number of cells occurs only through the first four or 

 five cleavages. This is exemplified in Nereis where regular doubling occurs 

 through the fourth cleavage, giving rise to sixteen cells. The fifth cleavage 

 results in twenty cells, the sixth in twenty-three, the numbers at successive 

 cleavages being twenty-nine, thirty-two, thirty-seven, thirty-eight, forty-one, 

 forty-two. The immediate cause of such irregularity in the number of cells is 

 that some of the cells divide more rapidly than others. Thus in Nereis after 

 the fourth cleavage not all the cells divide at any one time. In some cases the 

 amount of yolk present in the cell appears to influence the rapidity of division, 

 the cells containing the greater quantity of yolk dividing more slowly than those 

 containing less yolk. While this is generally true, exceptions where cells con- 

 taining much yolk divide as rapidly as, or more rapidly than, those containing 

 less, prove the inadequacy of this as a general explanation of numerical ir- 

 regularity in cleavage. It is possible that in many instances, at any rate, the 

 future role of the cell as regards function may play a large part in determining 

 the rapidity of its segmentation. After the forty-two-cell stage in Nereis the 

 number of cells at each successive cleavage is indeterminate, that is, it varies 



