140 FERTILIZATION 



this century. Even in 1900, for example, Vernon could report 

 twenty-nine different and successful echinoderm crosses; but 

 though these and subsequent cross-fertilization experiments are 

 interesting from an embryological and geneticai point of view, they 

 do not help in gaining an understanding of specificity and self- 

 sterility. In general terms the specific reaction between a sperm- 

 atozoon and an egg is due to complementariness of surface struc- 

 ture which allows close contact to occur over an area which, by 

 analogy with serological reactions, might be of the order of 

 100 A^; this enables the weak forces of intermolecular interac- 

 tion to combine to produce a strong union. The fit of an anti- 

 body to an antigen is close, the complementariness in structure 

 being such that an increase of o-8 A in the size of one atom in a 

 group can cause steric hindrance. On the other hand, cross- 

 fertilization does not imply that the heterologous spermatozoa in 

 question have surface combining configurations which are identical 

 with those on homologous spermatozoa. For example, in antigenic 

 proteins containing azobenzene arsonic groups, the substitution of 

 a methyl group by a chlorine or bromine atom causes little change 

 in serological properties, because the van der Waals' radii of the 

 methyl group, chlorine and bromine are, respectively, 1-9, i-8i 

 and 1-95 A. We can imagine that different groups in particular 

 regions of the folded polypeptide chains, which probably con- 

 stitute the protein parts of the reacting molecules, sometimes have 

 sufficiently similar van der Waals' radii in different species to allow 

 an unexpected cross-reaction to take place. The 'rubberiness' of 

 the fit, referred to in the jig-saw puzzle analogy, is reflected in the 

 behaviour of anti-^-(^-azobenzene-azo) benzene arsonic acid 

 antiserum, in which a radial dilation of about 1-5 A is possible 

 without interference with the reaction. 



To sum up, the only advances that have been made in the 

 study of specificity in fertilization since F. R. Lillie wrote a 

 chapter on this subject, which all students of fertilization should 

 read, in Problems of Fertilization, are: (i) more information about 

 the immunological nature of the reactions between the gametes, 

 almost entirely due to the work of Tyler; (2) more information 

 about the nature of immunological reactions in general, much of 

 which is due to Landsteiner and Pauling; and, possibly, (3) the 

 development of techniques for the quantitative study of inter- 

 specific and intergeneric cross-fertilization. 



