CELLS IN DIVISION 



extends equatorially from the poles of the cell where, presumably, 

 the cortex is less rigid. Chambers^^^ states that bubbling at the surface 

 of the dividing fibroblast follows this course; this observation I can 

 confirm from frequent observation. It was shown by Just^^' that, 

 Echinarachnius eggs when placed in hypotonic sea water during cleavage 

 burst most readily at the poles. Chambers^^^ finds that the sperma- 

 tocytes of the grasshopper Dissosteria during cleavage can be readily 

 provoked to extrude polar blebs by touching them at this point with a 

 microdissection needle; elsewhere on the cell surface this reaction does 

 not occur. A further point of resemblance between cleavage in a fibroblast 

 and in an egg is revealed in an observation of Jacobson (unpublished) 

 who has seen in cells in tissue culture before cleavage a narrow equatorial 

 band of basophilic cytoplasm comparable to the ring of cortical gel in 

 the Arbacia egg which marks out the site of the cleavage furrow. 



Thus, although the statement of Gray^^^ that 'no comprehensive 

 theory of disjunctive cell division has yet been put forward' is still true, 

 some points of similarity with geometrical cleavage have since become 

 clearer. Little, however, has since been learnt concerning the most 

 obvious feature of the process, namely the surface 'bubbling' during 

 cleavage. Of this the present writer is painfully aware, for in the course 

 of demonstrating films of mitosis in cells in tissue culture to scientific 

 meetings over a considerable period, it has been invariable that this- 

 aspect of the process attracts an undue share of notice, diverting atten- 

 tion from other points, and leads to questions to which no satisfactory 

 answer is yet possible. 



The phenomenon was first observed by Peremeschko,^^^ who noticed 

 an amoeboid movement of cell extrusions from epithelial cells of 

 Triton in cleavage, which he was observing in the living state. Study of 

 film records of the division of fibroblastic cells in culture reveals some 

 details of the 'bubbling' process. A clear bleb of cytoplasm is extruded 

 from the cell surface and assumes a rounded outline. For several 

 seconds it rapidly grows in size to a diameter of 3-5!^- and then the 

 process is reversed. The bleb decreases in size, usually at a slower 

 rate (Figure 56). There is a strong tendency for repeated 'bubbling' 

 at the same site. The blebs grow larger as cleavage of the cell proceeds; 

 the largest become virtually pseudopodial expansions of the cell wall. 

 Treatment of the culture with any of a number of chemical agents 

 exaggerates the degree of bubbling, some to a fantastic extent (p 196). 



It may be suggested that the bleb is formed by the escape of fluid 

 endoplasm through a local weakness in the cell wall, and that its 

 surface gelates in contact with the external medium. Outward flow 

 ceases when the pressure resulting from the elastic tension in the bubble 

 wall balances the general hydrostatic pressure in the cell ; further gela- 

 tion of the wall will result in an excess pressure within the bubble which 



145 



