THE MITOTIC CYCLE 



Such Streaming movements, demonstrated by the movement of gran- 

 ules, were observed in the eggs of small Nematodes* by von Erlanger^^^ 

 in 1897 and in several other eggs by Spek.^^^ They were believed until 

 recently to be analogous to the streaming movements seen at the 

 surface of an oil-drop when the surface tension is locally lowered. This 

 analogy was the main evidence for the theory that cleavage of the cell 

 was caused by a decrease in surface tension at the cleavage furrow. 

 It is now, however, certain that the surface of living cells is not a liquid 

 protoplasmic interface, and evidence from a number of sources has 

 shown that the tension at the cell surface is very low indeed; this 

 evidence is admirably reviewed by Danielli.^^^ The streaming move- 

 ments in the egg at cleavage are now regarded by Schechtman^^* 



Primary furrow 



Figure 54 Two stages of the ingrowth of 

 the cortex of the egg of Triturus torosus 

 during cleavage, revealed by experi- 

 ments with vital stains. Cortical pigment 

 black, remainder of cortex white; sub- 

 cortical cytoplasm cross-hatched. The 

 letters indicate the same sites in the 

 cortex at each stage. From Schecht- 

 MAN^^* {By courtesy, Science). 



and by Motomura^^* to be located below the gelated cortex. At the 

 egg surface, movement on a lesser scale does occur. Dan et alii^^^ 336 337 

 have shown that in several eggs at the time of cleavage a wave of 

 expansion spreads from the poles of the surface which is most marked 

 in the cleavage furrow (Figure 55). Swann and Mitchison believe 

 that this expansion begins at the poles because an active substance, 

 released from the chromosomes in anaphase, first reaches the cell sur- 

 face in these areas (p 127). The cleavage of the egg is thus a complex 

 process, involving the co-operation of several events, the relative 

 importance of which may well vary from species to species. Gray (1931) 

 was of opinion that the growth of the asters played a large part, for 

 as he says 'any irregularity in the size or position of these structures 

 is invariably accompanied by an irregularity in the form and position 

 of the cleavage furrow.' Painter^^^ found that if the formation of 

 the asters in the Arbacia egg were suppressed by the action of phenyl 



142 



