SPERM-EGG INTERACTING SUBSTANCES, II 45 



a methyl group does not inhibit attraction; Compound 4 deals 



HO-C 

 with the necessity or otherwise for the configuration I 



C-H 

 HOC 

 The role of the configuration I is dealt with by Compound 5. 



If an OH group is sufficient, without an H atom on C^ or C^, 

 Compound 6 should be effective. It would, perhaps, be preferable 

 to introduce bromine atoms rather than methyl groups in Com- 

 pounds 3-6 in Table 7. Unfortunately, Compounds 3 and 6 with 

 Br atoms replacing the Me groups are too unstable to exist in 

 aqueous solution. This type of experiment is capable of extension 

 in many interesting directions provided the required substances 

 can be made; but although the experiments are simple, certain pre- 

 cautions, which have not always been observed in the past, must 

 be taken. As mentioned earlier, the experimental procedure is to 

 fill a glass pipette with the test substance, put the pipette or one 

 end of it into the sperm suspension, and see what happens. If the 

 solution in the pipette is not a gel, spermatozoa may swim into the 

 pipette by chance and be unable to get out, because the diameter 

 of the pipette is too small to allow random sperm movement. 

 Failure to appreciate this difficulty has undoubtedly been respon- 

 sible for some of the claims for the existence of sperm chemotaxis 

 in the animal kingdom. Furthermore, convection currents and 

 hydrodynamic flow, or even a pumping and suction action, can 

 and do occur, and may cause spurious results, both of a positive 

 and negative kind. When using solutions of organic acids in agar 

 gels, attention must be paid to the possibility of the acid decom- 

 posing when the agar is dissolved by heating. An alternative 

 method of preparing the pipettes is to fill them with i % agar in 

 water and, after cooling, allow both ends of the pipette to dip into 

 aqueous solutions of the organic acid. The normal process of 

 diffusion will ultimately make the concentration of the acid in the 

 pipette equal to that in the solutions in which the two ends of the 

 pipette are immersed. For a pipette of fength 5 cm., it will take 

 three or four days for the average concentration in the pipette to 

 become half of what it is in the external solutions. The process 

 will not be seriously slowed up if carried out in a refrigerator to 

 avoid bacterial contamination. 



Returning to Table 9, a further interesting feature is the ability 



