CHAPTER 8 



STRUCTURAL CHANGES AT 

 FERTILIZATION 



Birefringence. Chapter i, The Morphology of FertiHzation, con- 

 tains a description of various structures which can be seen in 

 fertiHzed and unfertilized eggs, with an ordinary microscope. 

 Studies with the polarising microscope provide information from 

 which a few inferences can be made about the sub-microscopic 

 morphology of eggs, the scale changing from microns to angstroms. 

 The most interesting of these studies concerns the egg cortex. In 

 the unfertilized sea-urchin egg, this structure is negatively bi- 

 refringent with respect to the tangent and has a radial optical axis 

 (Runnstrom et al., 19446). The birefringence is very weak but as 

 the cortex also scatters and depolarises a considerable amount of 

 light, the egg surface appears quite bright under the polarising 

 microscope. At fertilization this negative birefringence disappears, 

 at about the same time as the light-scattering properties of the egg 

 surface change (chapters i & 9). Some minutes later, the cortical bi- 

 refringence re-appears, reaching a maximum by anaphase. Accord- 

 ing to Mitchison & Swann (1952), the intrinsic birefringence is 

 more strongly negative than the total birefringence, so that the 

 form birefringence must be positive. This suggests that the mole- 

 cular structure of the cortex is radial, but that the micellar organisa- 

 tion is tangential. The fall in total birefringence at fertilization is 

 probably due to a decrease in radial molecular order. 



Both the fertilization membrane and the hyaline layer are bi- 

 refringent. The former is virtually isotropic when first formed, but 

 by the time it is fully tanned (pp. 9-10), its form and intrinsic 

 birefringence are positive (Runnstrom, 1928; Runnstrom et al., 

 1946). The birefringence of the hyaline layer is positive with 

 respect to the tangent (Runnstrom et al., 19446). 



Mechanical properties of the cortex. Many of the methods of in- 

 vestigating changes in the physical state of the cortex and the 

 cytoplasm suffer from the disadvantage that they do not enable an 

 estimate to be made of the degree to which measurements of 

 cortical viscosity, surface tension, or rigidity are influenced by the 



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