m] DIVISION OF CYTOPLASM 41 



The actual division of the cytoplasm is very probably due 

 to diminished surface tension around the equator of the 

 cell at the greatest distance from the poles of the spindle 1 . 

 It can easily be shown by experiment that when the surface 

 tension of a sphere of liquid is reduced around its equator, 

 it will tend to divide along the equatorial plane. The rela- 

 tion of the formation of the division-furrow which initiates 

 cytoplasmic division to the position and size of the spindle 

 has been stated thus : the speed with which the appearance 

 of the cytoplasmic division-furrow follows the metaphase 

 of the mitosis and the depth to which it extends (in mero- 

 blastic eggs) are in general proportional to the length of the 

 spindle. That is to say, the shorter the spindle, the nearer 

 it must be to the surface of the cell if the furrow is to be 

 formed in a given time ; or if the spindle has a given length, 

 the deeper it lies below the surface the longer it will take to 

 produce a division-furrow. HERLANT has shown that, other 

 things being equal, the length of the spindle is greater if 

 there is more nuclear matter, so that indirectly the rate at 

 which cytoplasmic division occurs varies with the size of the 

 nucleus if the cytoplasmic volume is constant. 



Generally speaking, any bodies included in the cell, apart 

 from the nucleus, seem to be distributed purely mechani- 

 cally with the cytoplasm into the two halves of the cell as it 

 divides. A possible exception to this rule is found in the 

 mitochondria, to which a short reference has already been 

 made, but it will be convenient to postpone any further 

 account of them to the chapter on the development of the 

 germ-cells. 



1 R. S. LILLIE (1916), from experiments with segmenting eggs placed in 

 dilute sea water, concludes that the surface tension of the cell is increased 

 around the poles while remaining constant at the equator. This is of course 

 equivalent to a relative diminution around the equator. 



