76 PRINCIPLES OF EMBRYOLOGY 



is little movement of cortical pigment granules in the neighbourhood 

 of the poles of the spindle and there is no noticeable cortical expansion 

 there. However there is a considerable flow of the granules along the 

 furrow as it first appears near the animal pole of the egg. This movement 

 along the furrow would seem to indicate a contraction acting in this 

 direction and at this place. Moreover if the eggs are viewed from the side 

 it can be seen that just before division the whole cell heaps itself up so that 

 its vertical height increases (Fig. 4.9). This movement is accompanied by 

 an increase in the resistance of the cortex to deformation as measured by 



Figure 4.9 



Side views of cleavage in a newt's egg removed from its vitelline membrane 

 to show the 'rounding up' at the beginning of division. A, part-way through 

 the first division; B, between first and second division; C, beginning of 

 second division; D, second division completed. (After Selman and Wad- 



dington 1955.) 



the suction apparatus of Swann and Mitchison. This must involve a force 

 acting in the cortex, which might be either analogous to a surface tension, 

 or, more probably, due to an increase in the elastic constants of the mater- 

 ial. In any case, the rising up of the egg suggests a decrease in surface area 

 rather than an increase. It becomes rather unplausible, then, to attribute 

 much importance to cortical expansion in this case; we seem rather to 

 have to deal with a locaHsed contraction along the length of the developing 

 furrow. 



There is, however, almost certainly another factor involved. When 

 an amphibian egg cleaves, the surface between the two blastomeres is 

 almost unpigmented, and thus differs sharply from the cortex of the 



