52 J. GRAY. 



in the tube ; if this precaution is not observed it will be found that 

 repeated estimations of the resistance of the same eggs occupying the same 

 volume give very variable results. It was found possible, however, to 

 collect the eggs in such a way as to obtain uniform readings from repeated 

 observations. To ensure an equal distribution of the eggs is a matter of 

 great difficulty in the case of experiments dealing with resistances below 

 20 ohms ; above this, however, the difficulty can be overcome by patience 

 and repeated washings of the eggs. 



The volume of the eggs having been accurately determined by means of 

 the graduation on the tube, some of the sea-water was removed from the 

 tube and the electrodes placed in position and the resistance of the eggs 

 determined. 



The electrodes were then removed and the tube filled with fresh sea- 

 water (care being taken not to remove any eggs with the electrodes).* 

 One or two drops of a dilute emulsion of sperm were then added and the 

 tube inverted so as to distribute the eggs equally through the sea-water. 



After a minute the tube was allowed to stand in the water of the 

 thermostat, until the eggs had again settled to the bottom of the tube. 

 It was usually found that the fertilized eggs occupied a larger volume 

 than the same eggs unfertilized ; in such cases the tube was transferred 

 to a hand centrifuge and very gently centrifuged to the required volume. 

 The excess of sea-water was again drawn oif and the electrodes placed in 

 position and the resistance of the egg determined. The use of the centri- 

 fuge was avoided as far as possible for fear of injm'ing the eggs ; it was 

 found, however, that gentle use of the machine had no effect on the 

 resistance of unfertihzed eggs, or upon the subsequent development of 

 fertilized eggs. If, however, the unfertilized eggs were so compacted 

 as to require rather vigorous use of the centrifuge, the experiment was 

 discontinued after the estimation of the resistance of the fertilized eggs. 

 Such eggs were usually found to be crushed although not broken, and 

 when returned to abundant sea- water the large majority developed 

 normally ."j* 



After each estimation of the resistance of the eggs, the conductivity 

 tube was filled with fresh sea-water and immersed horizontally in sea- 

 water whose temperature never differed from that of the thermostat by 

 more than 1° C. 



Objections may be raised against the conclusion that the observed 

 differences in resistance of eggs at different stages of development are 



* When not actually in use the electrodes were kejit immersed in clean sea-water, 

 t These remarks apply to eggs which gave a resistance of 70 ohms and upwards. 



