178 



COMPARATIVE PHYSIOLOGY 



intensely active by addition of egg-water. There is some 

 indication — though the evidence is inconclusive — of specificity 

 in this reaction, as can be seen from the following table, which 

 summarises the effect of egg-water on sperm of different 

 genera of starfishes and sea-urchins : — 



However, it would not be justifiable to conclude from this 

 line of argument that the sperm is directed to the egg by the 

 excretion of substances from the latter whose diffusion sets 

 up a gradient in favour of greater motility in propinquity to 

 the egg itself. The question has been further attacked by 

 two forms of procedure known respectively as the drop and 

 tube methods. The latter v/as introduced by Pfeffer, and con- 

 sists of filling capillary tubes with egg-water or other fluid 

 and observing the reaction of the sperm when the tubes are 

 placed in a sperm-suspension. The data so obtained are 

 difficult to interpret. It is true, for example, that capillary 

 tubes containing sea water which has been in contact with 

 ripe eggs of Echinus esculentus soon become plugged with 

 sperm when introduced into a sperm-suspension of the same 

 species. And Dakin and Fordham (1924) have endeavoured 

 to establish the chemotactic orientation of the sperm towards 

 the egg by comparing the accumulation of sperm in the egg- 

 water tubes with accumulation of sperm in tubes containing 

 other immobilising agents {e.g. acid) which would act as a 

 trap collecting the quiescent sperm. With this particular 

 species the control tubes were always found to contain less 

 sperm than the egg-water tubes, and it is pointed out by the 

 authors mentioned that the sperm travelled a greater length 

 in the egg- water tubes. However, using the same method 

 for the study of other material, both BuUer and Loeb obtained 



