CHAPTER 3 



SPERM-EGG INTERACTING SUBSTANCES, II 



Morphology of chemotaxis. This chapter is concerned with the 

 chemotaxis of spermatozoa towards eggs, as a result of the secretion 

 of substances from eggs or the cells near them. Except possibly in 

 the case of the coelenterate Spirocodon saltatrix (J. C. Dan, 19506), 

 the phenomenon almost certainly does not occur, and certainly has 

 not been shown unequivocally to occur, in the animal kingdom. 

 But in plants and, in particular, in the ferns, mosses, horse-tails, 

 liverworts and quillworts, the chemotaxis of spermatozoa towards 

 egg secretions is an established fact. Chemotaxis means that 

 spermatozoa are attracted towards eggs through the medium of 

 some substance produced by the eggs or cells near them. The most 

 famous case of sperm chemotaxis, discovered by Pfeffer (1884), 

 occurs in the ferns. For example, the archegonia of bracken, 

 Pteridium aquilimim (Linn.), are said to produce L-malic acid * 

 which diffuses into the external aqueous medium. Bracken 

 spermatozoa are sensitive to the malic acid gradient produced by 

 the diffusion of this acid out of the eggs, and swim towards the 

 source. If there were no gradient, that is if the spermatozoa were 

 suspended in a uniform solution of malic acid, the spermatozoa 

 could not swim preferentially in any particular direction and would 

 remain uniformly distributed. Before considering the chemistry 

 of chemotaxis, it is worth making a brief examination of the mor- 

 phology of the reaction. The behaviour of bracken spermatozoa in 

 ordinary tap water is shown in Fig. 8fl. This diagram was obtained 

 by taking a cinematograph film of the spermatozoa swimming in 

 tap water, projecting the film frame by frame on to paper, and 

 joining up the consecutive positions taken up by each spermatozoon 

 by straight lines, so that every spermatozoon in the field is asso- 

 ciated with a 'track'. The movements of the spermatozoa will be 

 seen to be random, in the sense that the tracks do not point in any 



* Although it is virtually certain that the substance is L-malic acid, chemical 

 identification has not yet been achieved. Prof. E. C. Slater and I tried, un- 

 successfully, to inhibit the reaction with malic dehydrogenase; but there was 

 evidence that the enzyme preparation was not sufficiently strong to decompose 

 the malate at the required rate. 



D 39 



