124 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



ing of the cortical gel structure (fig. 13 ) . These measurements of the struc- 

 tural state of the gel system were made upon eggs which were about to di- 

 vide (5 min, prior to furrowing). At this time the gelational state of the 

 cortical cytoplasm had already risen sharply to its high maximum, and 

 a centrifugal force of 25,000 X g was required to achieve a good displace- 

 ment of the cortically embedded pigment bodies within a reasonable 

 l^eriod of centrifugation. 



GENERAL CONCLUSIONS 



All in all, the various experiments here reported seem to jirovide sub- 

 stantial support for the hypothesis that gel structures formed by the cell, 

 particularly the cortical plasmagel layer of the cytoplasm, are intrisically 

 contractile and thus are instrumental in the performance of mechanical 

 work. Metabolic energy, diverted into the endothermic gelation process, 

 apparently, reappears in the form of mechanical work, the work of 

 cytokinetic furrowing, when the fibrillar protein components of the gel 

 structures undergo a forceful folding — thus reconverting the protein units 

 into the mode globular form which is charcteristic of the sol condition. 



Apparently the strength of the intermolecular linkages of the gel struc- 

 ture is a critical factor in determining the strength of the contractile force 

 that it can develop. In any event, under widely varying experimental con- 

 ditions and with several different kinds of cells it has been found that 

 whenever the tensile properties of the plasmagel structure are weakened, 

 the force of the mechanical movements of the cell is porportionately re- 

 duced. And conversely, all treatments that fortify the plasmagel struc- 

 ture have been found to give a corresponding increase in the mechanical 

 performance. 



Some evidence is now on hand which indicates that the high potential 

 phosphate bonds of adenosine triphosphate may be utilized by the cell 

 in deriving energy for the building of its gel structures. However, further 

 studies are needed on the metabolic aspects of protoplasmic gelations. 



SUMMARY 



A survey of the evidence bearing on the mechanism of cytokinesis in 

 marine eggs provides firm support for the hypothesis that cleavage results 

 from the contraction of the strongly gelled cortical cytoplasm (plasmagel 

 layer) , first in the equatorial region and then in the walls of the deepening 

 furrow. The strength of the furrowing reaction appears to be determined by 

 the structural state of this plasmagel layer, as measured by the centrifugal 

 method. Whenever the structural strength of the plasmagel is weakened to 

 a critical degree, by high pressure or by low temperature, acting singly or 

 in combination, furrowing is aborted and cleavage fails. Also, lesser de- 



