STRUCTURAL CHANGES AT FERTILIZATION 93 



suggesting a layer of the order of 100 A thick for this part of the 

 cortex. If magnified, a sea-urchin egg would therefore be more like 

 a tennis ball, which resists deformation because of the rigidity of 

 its 'cell membrane', than like a rubber balloon, which resists de- 

 formation because of its internal pressure. Measurements are ex- 

 pressed in terms of corrected stiffness, which involves correcting 

 observations for variations in egg and pipette diameters. Full 

 details are given in Mitchison & Swann (1954a). Experiments 

 with the Elastimeter show that the Young's modulus of the cortex 

 of the unfertilized sea-urchin egg is 1-2. 10* dynes. cm"^, and that 

 the internal pressure of the egg is less than 95 dynes. cm"^ (Mitchi- 

 son & Swann, 19546).* These values agree rather well with those of 

 E. N. Harvey (193 1), who measured the centrifugal force necessary 

 to split a sea-urchin egg into two halves. 



What happens, structurally, to the cortex at and after fertiliza- 

 tion? Mitchison & Swann (1955) found that, at fertilization, there 

 was a sudden rise in stiffness, from the value 9 dynes. cm~2./x~^ for 

 the unfertilized egg, which was followed by a fall, during the early 

 sperm aster stage, to the lowest value, 4 dynes. cm"-./x~^, which 

 occurs during development. There is a steep rise, to about 61 

 dynes. cnr-.ju,"^, in late anaphase, just before cleavage. Normal eggs 

 cannot be used for direct measurements of stiffness at fertilization, 

 because of the elevation of the fertilization membrane. Runnstrom 

 et al. (19446) discovered, however, that treatment of unfertilized 

 sea-urchin eggs with trypsin prevented the elevation of the fer- 

 tilization membrane, perhaps by digesting the vitelline membrane. 

 Treatment of unfertilized eggs of Psammechinus microtuberculatus 

 (de Blainville) with o-i% w/v. trypsin in sea water reduced the 

 stiffness of the cortex of these eggs, which normally is 9 dynes. 

 cm~^.ju,~^, by a factor of 4 or 5. At fertilization, the cortical stiffness 

 increased by the same factor. Fig. 17. 



In their most recent paper, Mitchison & Swann (1955) give a 

 comprehensive review of the earlier methods of investigating the 

 mechanical properties of the cortex. More or less serious objec- 

 tions can be raised against all the classical methods. Cole's experi- 

 ments have already been discussed ; in the same way, measurements 

 of the ease with which eggs can be separated into two halves by 



* Young's modulus can be derived from the corrected stiffness values, assum- 

 ing a cortical thickness of i 6 ft and zero internal pressure. The internal pressure 

 is very low. 



