SPECIFICITY 137 



time of writing this book, the method has not been appUed to cross- 

 fertiHzation experiments, the unfertihzed eggs simply being left in 

 contact with the heterologous spermatozoa for an unknown and 

 indefinite time. 



(3) Increase p. p can sometimes be increased by removal of egg 

 jelly (Harding & Harding, 1952a), by pre-treatment of the gametes 

 with glycine + egg water or sodium periodate -\- egg water 

 (Harding & Harding, 19526), or by interfering with the vitelline 

 membrane of unfertilized eggs through treatment with such agents 

 as trypsin(Hultin, 1948). Trypsin alsoinduces polyspermy in homo- 

 logous fertilization, as Hagstrom & Hagstrom (1954c) have recently 

 shown. One might envisage the surface of an unfertilized egg as a 

 three-dimensional jig-saw puzzle, made of rubber and containing 

 very weak magnets. Of course, the magnets are, in reality, van der 

 Waals' forces which are inversely proportional to the seventh 

 power of the distances between the atoms involved, hydrogen 

 bonds and attractions between oppositely charged groups (Pauling 

 et al., 1943). The application of trypsin could be likened to a 

 bunsen burner turned on to the jig-saw puzzle. Some of the pro- 

 tuberances on the surface will be melted, decreasing the preciseness 

 of fit with complementary structures on the head of the homologous 

 spermatozoon, but permitting a reaction to occur with less precisely 

 complementary structures on the heads of some heterologous 

 spermatozoa. An alternative interpretation of the effect of trypsin 

 is, however, possible — that it exposes more or 'deeper' combining 

 groups. Incomplete antibody, for example, combines with antigens 

 on red blood cells, though no agglutination takes place. But if the 

 red blood cells are pre-treated with trypsin, washed, and then sub- 

 jected to incomplete antibody, agglutination occurs (Coombs, 

 1954). This suggests that, so far from blunting combining groups, 

 treatment with trypsin sharpens or exposes them. A similar con- 

 clusion might be reached from the work of Coffin & Pickles (1953), 

 who found that periodate destroyed the D P.h antigen and that 

 subsequent treatment with trypsin brought back the property of 

 agglutination. The procedure could be repeated, as if combining 

 sites were obliterated by periodate, but that new ones were exposed 

 by a second treatment with trypsin. 



Runnstrom et al. (19446) found no improvement in cross- 

 fertilizations between Psammechimis miliaris and Echinocardium 

 cordatum after pre-treating the unfertilized eggs with trypsin, 



K2 



