CELLS IN DIVISION 



Cortical changes 



The cortex of the sea-urchin tgg is a structure of great importance 

 and interest. By studying the effects of temperature and high pressure 

 upon it, Marsland^^^ has shown that cortical gelation is an endother- 

 mic process, and the energy needed for cleavage is stored in the gelated 

 cortex previous to this period of mitosis.* It is to be hoped that Mars- 

 land will tell us whether the same is true of the mitotic spindle. 



In the Arbacia ^gg, the pigment granules migrate into the cortex 

 within ten minutes after fertilization (Harvey^^'^) and are not displaced 

 therein by the centrifugal forces which suffice to statify the yolk 

 granules in the inner zones at ordinary pressures. Marsland's^^^ 

 observations on this point show that at the equator the cortex is 

 especially rigid : 



Although it is evident that the plasma gel over the entire surface of the egg 

 undergoes a setting process just prior to and during cleavage, some evidence has 

 appeared . . . which indicates that a maximum rigidity is present in the equatorial 

 region. When eggs are centrifuged for a long period ( lo minutes) at 68 atmospheres, 

 there is some tendency for the pigment granules of the poles to be dislodged cen- 

 trifugally, whereas those in the equatorial region remain fixed. This leaves a 

 fairly conspicuous pigment band girdling the equator of the egg. 



Cornman and Cornman^^'^ suggest that a 'furrow-determining sub- 

 stance' released by the nucleus at the end of prophase is carried in a 

 nearly symmetrical pattern to the cell cortex'. The plane of the cleavage 

 furrow may be determined at a time even before this, for E. B. Harvey^^^^ 

 found that if the nucleus of the ^gg of Parechinus was displaced by 

 centrifuging at a stage before the dissolution of the nuclear membrane, 

 the egg cleaved nevertheless in a plane through the original position 

 of the nucleus. 



In an apparatus described by Brown^^^ eggs can be centrifuged 

 while under pressure, and in these circumstances the pigment granules 

 in the egg cortex can be displaced, both at the equator and elsewhere. 

 Brown has compared the readiness with which they move at various 

 times after fertilization, and has constructed a cortical viscosity 

 curve at 408 atmospheres which can be compared with that of 

 Heilbrunn for the inner zones at ordinary pressures, for which the 

 absolute rigidity is enormously lower (Figure 52). The interesting fact 

 emerges from this comparison that there is an increase in cortical 

 rigidity some time before the rise in general viscosity during cleavage; 

 before and during cleavage therefore, when permeability of the surface 

 is high, the cortex of the tgg is especially stiff. It is of interest to com- 

 pare this state of affairs with that which immediately follows fertiliza- 

 tion, where again permeability is high, and the whole egg has become 



* Zeuthen'^'* has shown that when cleavage is in progress in the eggs of Psammechinus and 

 Urechis, the rate of oxygen uptake is then decreasing. It rises again during the next mitotic 

 cycle. 



