CLEAVAGE Jl 



forward mechanical part in carrying the process through to completion. 

 It seems that the main active agent must be the cortex itself. 



Probably the most widely accepted theory is one which ascribes the 

 cleaving of the cell to the contraction of a ring of cortex around the posi- 

 tion of the future furrow. This theory, which has recently been defended 

 by W. H. Lewis (195 1) and Marsland (195 1, Marsland and Landau 1954) 

 is the one which suggests itself most naturally when one looks at dividing 



5-2.5 



5-42 



5.50 



Figure 4.6 



The independence of the cleavage furrow, once it has started, of contact 

 with the spindle. In a newt's egg which was just starting to cleave at 3.23 

 p.m., a strip of cellophane was inserted under a region through which the 

 furrow should extend. Within the next half-hour it did actually extend 

 through the area. (From Waddington igs^d) 



cells and tries to think how their behaviour might be explained in terms 

 of cortical activity. There is, however, rather little direct evidence for it. 

 And, as Mitchison (1952) points out, there are some difficulties in it when 

 one examines the matter more closely. In the first place, if the cell is to 

 be constricted completely into two parts, the ring of contracting material 

 would have to contract away to nothing. This, Mitchison argues, would 

 seem to be an unlikely event, if one thinks of it in terms of a contraction 

 like that of muscle: it is perhaps not so unplausible if one pictures the 

 contracting ring as similar to the ectoplasm at the posterior end of an 

 advancing amoeba, which liquefies after it has finished contracting. Again, 



