DOUGLAS MARSLAND 



113 



CYTOKINESIS 



Background. Normal cell division unquestionably represents a com- 

 plex series of events and processes. Here, however, we shall deal mainly 

 with the furrowing process, by which the animal type of cell divides 

 itself into two daughter cells. The evidence to be reviewed represents a 

 very thorough testing of the cortical gel contraction theory of cytokinesis. 

 This theory, as formulated by Marsland (21), postulates that the fur- 

 rowing j)otcncy in animal cells depends upon the structural state and 

 hence tiie contractile cajmcity of a strongly gelated cortical layer of 

 cytoplasm, especially in the furrowing region. Thus the basic mecha- 

 nisms for the development of mechanical energy in cytokinesis and in 

 amoeboid movement are thought to be essentially the same. 



Fig. 1. Plasmagel strength 

 and rate of progress of fni'rows 

 in i-elation to liigh pre.ssure. 

 Data from Marsland, 1938. 



100 ; 



80 



60. 



W 



40 



> 



20 



ARBACIA PUNCTULATA 

 23" 



_»_,_ RATE OF FURROW PROGRESS 

 _0-0- PLASMAGEL STRENGTH 



3 4 5 



PRESSURE. Ibs./in.2 



6xl05 



The existence of a cortical plasmagel layer in an egg-cell {Arbacia 

 punctulata) was first demonstrated clearly by the experiments of Dugald 

 Brown in 1934 (1) . These experiments showed that the cortically embedded 

 pigment bodies (red chromatophores) are considerably more resistant to 

 centrifugal displacement than those in the deeply-lying (medullary) 

 cytoplasm of the egg. And in addition Brown showed that this well- 

 defined plasmagel layer, like that of the Amoeba, is susceptible to solation 

 under pressure; and that the plasmagel layer of the egg becomes very 

 much more firmly gelled prior to and during the furrowing period. 



Pressure Effects in Dividing Eggs. The main effects of pressure on 

 the gel structure and furrowing performance of Arbacia eggs during first 

 cleavage are summarized in figure 1. Here it may be seen 1) that an 

 exponential weakening of the cortical gel structure occurs as the pressure 

 increases; 2) that the progress of the furrow as it impinges on the spindle 



