140 The Structure of Protoplasm 



does not participate in the flow of escaping plasmasol, but also in its 

 behavior when manipulated with micro-needles. Finally, the work 

 of Marsland, '38 and '39a, has confirmed and amplified the original 

 observations of Brown. 



(2) The setthig of the cortical gel diLring active cleavage 



An indication that the furrowing process might involve gelation 

 reactions occurring primarily in the cortical layer of the egg cell was 

 provided by Brown's observation ('34c) that when the furrows are 

 about to appear (and during the time when they are actually 

 cleaving the cell) , the plasmagel part of the protoplasm becomes set 

 much more firmly than in other phases of the division cycle (see Fig. 

 5) . This very marked shift in the equilibrium is indicated by the fact 

 that no displacement of the pigment granules of the cortex can be 

 obtained even in a relatively strong centrifugal field (18,000 X 

 gravity, Marsland, '39a) when the eggs are centrifuged during the 

 cleavage period; whereas, if the centrifuging is done shortly before 

 or shortly after this period, a clear displacement can be obtained 

 in relatively weak fields (7,000 X gravity) . 



On the basis of the foregoing observations, experiments were 

 undertaken to determine the effects of pressure upon the visible 

 aspects of cleavage in the Arbacia egg-'' (Marsland, '38) , and upon 

 the sol ^gel equilibrium of the plasmagel system (Marsland, '39) . 



(3) The cleavage block at high pressures 



The first type of experiment involved subjecting the eggs to a 

 relatively high pressure (7,000 Ibs./in.-) applied suddenly at the 

 time when fairly deep furrows were visible in a majority of the cells. 

 As soon as the pressure is applied, the progress of each furrow 

 ceases. This is true whether the invagination has just begun or 

 whether it has already almost cleaved the egg. Not only does the 

 inward movement of the furrow stop, but soon a slow recession 

 begins (Fig. 6) and if the pressure is maintained, each of the 

 bilobed cells gradually-^ reverts to a sphere. 



It seemed likely that this phenomenon might be comparable to 

 the slow rounding of amoeboid cells which occurs at a similar pres- 

 sure. Apparently the bilobed form of the cleaving egg is stable 

 provided that the cortical plasmagel possesses a sufficient degree of 

 rigidity. According to this view the cells become rounded under 

 the agency of surface forces as soon as, due to the solating effect of 

 the pressure, the resistance of the plasmagel drops below a certain 

 critical value. 



The inhibition of furrowing by pressure is remarkedly reversible. 



