CELLS IN DIVISION 



for Mazia^^'^ did not extend his observations beyond fertilization. 

 MiRSKY states that there is no further change in the solubihty of 

 the egg proteins for two hours after this point. However, one piece of 

 evidence of some importance can be cited. Shapiro^^^ has shown that 

 Arbacia eggs undergo cleavage more rapidly in the absence of calcium 

 in the medium and more slowly when an excess of this ion is added. 

 This observation is thus consistent with the view that calcium is 

 normally withdrawn from the surface of the tgg before cleavage. 

 In other types of cell, the order of events may well differ from those in 



fV2 



Ml 



1-W 



1-39 



1-38 



20 VO 60 



Minutes from ferfil/'zafion 



80 



100 



Figure 53 Changes in the refractive index of the whole egg of the sea 

 urchin, from fertiUzation to the ist cleavage. From Vles.^^^ 



the Qgg', the evidence from plant cells, such as it is at present, does not 

 suggest that during the mitotic cycle a high internal viscosity coincides 

 with an increased permeabihty. 



Cleavage in animal cells 



The part played by the gelated cortex in the cleavage of the egg ceil 



has received prominence in recent years. 



Schechtman^^^ showed that in the eggs of the Newt Triturus torosus, 

 the first event in cleavage is an equatorial infolding of the egg surface; 

 this was demonstrated by the fact that marks made on either side of the 

 equatorial great circle were invaginated within the egg (Figure 54). The 

 cortical furrow which is formed grows by the gelation of subcortical 

 cytoplasm which flows beneath the cortex into the equatorial plane. 



* The Nematode egg appears to provide a remarkable combination of an impermeable 

 surface with a cleavage 'uniquely resistant to the effects of hydrostatic pressure' (Pease and 

 Marsland'^ with reference to Ascaris) . 



141 



