94 FERTILIZATION 



centrifugation are open to the criticism that the measurements are 

 sensitive to changes in cytoplasmic gelation as well as to the struc- 

 tural changes in the cortex, which are the object of the enquiry. 

 In spite of difficulties in estimating the internal pressure of eggs, 

 upon which interpretation of Elastimeter measurements depends 

 to a considerable extent, this technique provides a pov^erful and 



lA 

 Hj 

 C 



V, 

 -to 



tj 



O 



O 



5 10 



t (min.) after fertilization 



Fic. 17. — Changes in cortical stiffness (corrected, in dynes. cm~^ per fi deforma- 

 tion), at fertilization. The four curves refer to four eggs of Psammechinus 

 ftiicrotuberculatus, previously treated with tr>'psin to prevent elevation of the 

 fertilization membrane. A, value for stiffness of normal unfertilized egg; B, 

 value for stiffness of trypsin-treated unfertilized egg; C, stiffness of 

 fertilized egg at sperm aster stage. After Mitchison Sc Swann (1955). 



sensitive new method of investigating the structural characteristics 

 of the cortex. 



Calcium and protoplasmic consistency. Heilbrunn (19 15, 1952) 

 has for many years been the principal proponent of the view that 

 cellular reactivity in general and activation in particular are 'caused' 

 by the release into the cell interior of calcium, previously bound in 

 the cortex in Ca-protein complexes. Once released, calcium pro- 

 motes a protoplasmic clotting reaction or gelation. The release of 



