m] NUCLEAR DIVISION AND CELL DIVISION 39 



especially the position of the nucleus in the cell, and the 

 relation of nuclear division to the division of the cytoplasm 

 which normally follows it. 



Since the plane in which the cell divides is perpendicular 

 to the axis of the spindle, it follows that the position of the 

 spindle determines the plane of cell-division, and the posi- 

 tion of the spindle in turn depends on that of the nucleus. 

 Generally speaking, at least in embryonic cells or those 

 which are not highly differentiated, the nucleus occupies a 

 position which may be regarded as the centre of gravity of 

 the cytoplasm. A spherical cell in which the cytoplasm is 

 uniformly distributed has the nucleus at the centre; but if 

 there are considerable masses of other substances present, 

 for example yolk, and if, as often happens, these are 

 arranged in zones so that the greater part of the cytoplasm 

 is at one pole and the rest of the cell is largely occupied with 

 yolk, then the nucleus lies at the centre of mass of the 

 cytoplasm independently of the yolk or other inclusions. 

 When a cell of this kind is about to divide, the spindle is 

 formed in such a way that its axis lies along that of the 

 greatest mass of cytoplasm, with the result that in general 

 when the cell-division takes place the cytoplasm is about 

 equally divided between the two daughter cells. This 

 happens irrespectively of the yolk or other inert substance, 

 so that a cell may divide into two very unequal parts, one 

 containing little except cytoplasm and the other a similar 

 amount of cytoplasm and a large amount of yolk, as regu- 

 larly takes place in the third cleavage division of a yolk- 

 laden egg like that of the Frog. The way in which the mass 

 of the cytoplasm influences the position of the spindle and 

 the consequent plane of cell-division is illustrated by the 

 results of compressing yolk-laden eggs with glass plates. If 

 the egg, with the cytoplasmic pole uppermost and the yolk 



