THE MITOTIC CYCLE 



egg sets up differences in the concentration of metabolites between 

 various parts of the egg. Such differences result in diffusion of particular 

 substances; these diffusion currents in turn exert forces on the cell by 

 an unequal molecular bombardment and by a viscous drag. These 

 tend to deform the cell, while the surface forces in the membrane oppose 

 any changes from the spherical shape ; if the diffusion forces are greater 

 than those at the surface the cell becomes unstable and elongates. 

 Elongation is followed by cell division (Buchsbaum and Williamson^**) . 

 From this basic reasoning, these authors derive equations for both the 

 axial elongation, and the equatorial constriction of the dividing egg, 

 and compare their theoretical curves with data obtained from photo- 

 graphic observations of the dividing Arbacia egg. The agreement be- 

 tween theory and practice is remarkably close for the elongation during 

 cleavage, though less so for the constriction of the cleavage furrow. 

 However, it is hardly possible for us to conclude that the structural 

 features of the egg cortex, spindle and asters do not play an essential role 

 in the process of division. That it is unlikely that metabolic factors alone 

 are predominant in cleavage is suggested by other evidence. Gallan'*^ 

 has shown that in four species of sea-urchins from Naples, there is no 

 correlation between cleavage rate and oxygen consumption. According 

 to MooRE,^*^ eggs deprived of part of their cytoplasm cleave at the 

 normal rate; it is hardly to be expected that the concentrations of 

 metabolites within an egg fragment would be identical with those in a 

 whole egg. Several agents are known which both inhibit cell division 

 and at the same time increase the rate of respiration in the sea-urchin 

 egg (Krahl and Clowes ;3*' Clowes and Krahl^*^). Meanwhile, 

 the results of Buchsbaum and Williamson remain to be explained if 

 they are due neither to coincidence nor to the forces which these 

 authors have postulated. 



The geometrical cleavage of the egg seems to provide a contrast to 

 the type of cell division seen in such irregularly shaped cells as the tissue 

 culture fibroblast. This latter type of cleavage Gray^^^ calls 'dis- 

 junctive'. The non-regular form of the dividing fibroblast is correlated 

 with the small size of its asters (Hughes and Swann^*^). The daughter 

 fibroblasts clearly separate by an active pulling apart ; and the stretch- 

 ing of the connecting stalk between the two cells leads to a secondary 

 birefringence of the interzonal region of the spindle (Hughes and 

 SwANN^*^), which is not seen in the dividing egg. A further point of 

 difference is the characteristic 'bubbling' at the surface of the fibro- 

 blastic cell in cleavage; this, however, does not provide a fundamental 

 distinction for several reasons. 'BubbHng' can occur in a spherical cell, 

 as in the egg ofSabellaria during the maturation divisions (Novikoff^^^) ; 

 the process has also been observed in the dividing spermatocytes of 

 grasshoppers and crickets by Baumgartner.^^^ Secondly, the process 



144 



