STRUCTURAL CHANGES AT FERTILIZATION 95 



calcium from the egg cortex is associated with the so-called lique- 

 faction or decrease in rigidity of this structure, which follows 

 fertilization or parthenogenetic activation. Heilbrunn believes that 

 this rigidity is due to the presence of calcium in the cortex; but 

 according to Mitchison & Swann (1955), removal of calcium from 

 the external medium makes very little difference to the rigidity of 

 the cortex of the fertilized sea-urchin egg (bearing in mind that 

 this treatment removes the hyaline layer), while in the case of the 

 unfertilized egg, calcium lack causes a slight, but insignificant, in- 

 crease in rigidity. W. L. Wilson ( 1 95 1 ) examined the post-fertiliza- 

 tion changes in the cortical properties of the eggs of Chaetopterus 

 variopedatus. He expressed rigidity in terms of the centrifugal 

 force, which, applied for i minute, was necessary to break the 

 continuous layer of granules which exists in the cortex of this egg, 

 in not less than i6 and not more than 19 eggs, out of a sample of 

 20. The experiment was done at different times after fertilization. 

 Wilson found that the rigidity of the cortex decreased after fertiliza- 

 tion, for about 4 minutes. After 6 minutes, the rigidity began 

 to increase and in lo-ii minutes, it had reached the pre-fertiliza- 

 tion level, where it stayed until 35 minutes after fertilization, when 

 it again declined. These observations do not fit well with the re- 

 sults of the more refined measurements of Mitchison and Swann, 

 though this may be explained by the different maturation states 

 of the eggs of Chaetopterus and of the sea-urchin at fertilization. 

 Alternatively, Wilson may have been measuring the Shear mo- 

 dulus, and not Young's modulus, which Mitchison and Swann 

 measured. Until 30 minutes after fertilization, Wilson's results 

 are to a certain extent reflected in the changes in cytoplasmic 

 viscosity * which occur in the same egg during the same time. 

 Heilbrunn & Wilson's results (1948) on this latter subject, which 

 are, perhaps, less open to doubt than those of Wilson on cortical 

 changes, are reproduced in Fig. 18. Similar curves were obtained 

 for the eggs of Arhacia punctulata and Cumingia tellinoides by 

 Heilbrunn ini92i. It is not quite clear how the observed reduction 

 in cytoplasmic viscosity, immediately after fertilization, fits in with 

 Heilbrunn's thesis that one of the important characteristics of egg 

 activation is cytoplasmic clotting or gelation, due to the release of 

 calcium from the cortex. According to Heilbrunn, a wide variety 



* The cytoplasmic viscosity of an unfertilized egg is about 3 times that of 

 water (Heilbrunn, 1952). 



