DOUGLAS MARSLAND 



117 



that the cell membrane is pulled inward into the furrow by a contraction 

 of the subjacent plasmagel in the equatorial region (12-14, 17, 20, 21). 

 Here, however, only the second viewpoint will be presented. 



As a prelude to division, before the cell elongates or displays any 

 definitive furrow, a structural gradient must develop in the plasmagel 

 system, which initially seems to be strongly and uniformly set throughout. 

 Cytokinesis is initiated, apparently, by a localized solation which weakens 

 the gel structure at each pole, as is indicated in figure 6B. Such a polar 

 weakening is revealed by the experiments of Edward Chambers (personal 

 communication) in which it was found that early telophase eggs exposed 

 to hypotonic swelling always burst at the poles. Also it is found that 



Fig. 5. Effects of temperature 

 and pressure on the structural 

 state of the plasmagel layer of 

 the Arbacia egg in relation to 

 cleavage capacity. Note that 

 furrowing is aborted whenever 

 the plasmagel strength falls be- 

 low a critical level, as a result 

 of the various temperature-pres- 

 sure combinations. The centri- 

 fuge times are given directly, in 

 seconds. Data from Marsland, 

 1950. 



200 



o 100 



Pressure/Temperciture 

 where division can occur 



Pressure/Temperature 

 where division can not occur 



Eggs of arbacia punc'ulata 



12 3 4 5 6 



p...r,„„/trii)0 ib,/sq. in 



the pigment bodies at the poles, compared to those in the equatorial zone, 

 are more readily displaced when early telophase eggs are subjected to high 

 force pressure-centrifugation. This polar weakening, apparently, permits 

 the more strongly gelled band-like portion of the plasmagel system in 

 the broad central zone to contract, forcibly bulging the cell out at the 

 poles as the cell undergoes elongation, and stretching the cell membrane 

 in the polar regions. Such a polar stretching seems quite plain in the 

 dividing neuroblasts depicted by Roberts (28), although in this case 

 one pole appears to display solation slightly before the other and Roberts 

 attributes the polar bulging to an active expansion rather than to a 

 passive stretching. At present, however, there does not seem to be any 

 decisive evidence which makes it necessary to assume that an active 

 process of expansion provides energy for furrowing; and in the absence 



