72 PRINCIPLES OF EMBRYOLOGY 



when a cell divides its surface area must increase; in the case of a spherical 

 egg becoming converted into tv^o spheres, the increase is about 26 per 

 cent. A contraction of the ring of the furrow would therefore have to 

 be compensated by a large increase in area elsewhere. The main evidence 

 on which Marsland rehes, for support of the 'contracting ring' hypothesis, 

 is the demonstration that if gelation of the cortex is prevented (by high 

 hydrostatic pressure or low temperature) the development of the cleavage 

 furrow is inhibited. This might be due, as he suggests, to an abolition of 

 the contraction; but it might equally be a consequence of the failure of 

 the ungelated cortex to transmit the stress from the expanding regions 

 to the furrow in the way required in the mechanism postulated by 

 Mitchison and Swarm. Thus the evidence is not fully conclusive. 



Mitchison and Swann (1955, see Mitchison 1952) have recently turned 

 the conventional theory upside down and suggested that the prime 

 mover in cell cleavage is not a contraction of the furrow region but an 

 expansion of the other parts of the cortex. They suppose that the cleavage 

 process is initiated by substances released by the two separating groups of 

 chromosomes (there seems no reason why one should not attribute the 

 activity to the centromeres, rather than to the chromosomes themselves, in 

 accordance with the points made on p. 66), This substance would diffuse 

 out into a more or less dumbbell-shaped region which, in a spherical egg, 

 would reach the cortex first at the equator opposite the poles of the spindle 

 (Fig. 4.7). The substance is supposed to cause an expansion of the cortex, 

 and this would begin in the same region. Meanwhile the ring of cortex 

 which hes in the plane of the equator of the spindle (and therefore in- 



FiGUM 4.7 

 Diagram of cleavage in die sea-urchin. (After Mitchison 1952.) 



