230 GENERAL EMBRYOLOGY 



correspond with the center of the entire cell. (2) The chief 

 axis of the mitotic figure tends to lie in the longest axis of the 

 protoplasmic mass. The result of this is that in cells that are 

 approximately spherical and homogeneous with respect to yolk 

 content, successive cleavage planes tend to alternate at right 

 angles with one another, for it would always be the longest axis 

 of the cell that is divided, and in most cases any other axis 

 would be greater than one-half the longest and no two successive 

 spindles would be parallel. The regular alternation of cleav- 

 age planes probably depends, as a matter of fact, upon a more 

 fundamental relation, namely, the position of the centrosome. 

 At the conclusion of a mitosis the centrosome lies at one end 

 of the axis passing perpendicularly to the plane of division; 

 when the centrosome divides, its halves usually migrate sym- 

 metrically to opposite sides of the nucleus, occupying the poles 

 of an axis lying parallel with the plane of the preceding division, 

 and since division always occurs at right angles to the axis 

 connecting the centrosomes, the plane of one division will be 

 at right angles to that of the preceding or succeeding cleavage 

 (Fig. 24). Any other relation between successive cleavage 

 planes involves either a change in the relative position of the 

 centrosomes, or a rotation of the cleavage spindle after its 

 formation. Thus in the formation of a simple epithelium, 

 where successive cleavages are nearly parallel, the centrosomes 

 migrate through approximately 90 at some time during the 

 interkinesis; and in the cleavage of some ova encased in com- 

 paratively rigid shells, the position of the spindle may change 

 (Lepas, Bigelow). 



Balfour's law of cleavage, which is really a corollary of 

 the first part of the Sachs- Hertwig laws, concerns the rate 

 rather than the geometrical relations of cleavage. This law 

 states that the rate of cleavage is inversely proportional to the 

 amount of deutoplasm contained within the cell. It follows 

 from the fact that the nucleus tends to lie in the center of the 

 protoplasmic mass, that in the unequal division of cells con- 

 taining localized deutoplasm, the smaller cell will contain 

 relatively a smaller proportion of yolk than the larger cell, and 



