Processes of the Echinoderm Egg during Fertilisation. 415 



The flasks during the course of work were recalibrated from time to time, 

 and at long intervals were tested by the liberation of a known amount of 

 heat in each flask from a small coil of constantan wire. This coil liberated 

 21 grm. -calories of heat per hour in the flask under the conditions of the 

 test, and the galvanometer scale readings were usually within 3 per cent, of 

 this value. To close the mouths of the flasks during an experiment, it was 

 found that thick wads of cotton wool were the most effective. When the 

 flasks were closed by rubber stoppers and the flask sunk completely in the 

 water of the thermostat, it was found that more heat was lost by conduction 

 through the stopper than was the case when they were only plugged with 

 cotton wool and sunk up to their necks in the water of the bath. The 

 flasks were mounted in pairs in open wirework baskets, which were made so 

 that they could be clamped on the thermostat, so the flasks were held firmly 

 submerged up to within a centimetre of the tops of their necks in the water 

 of the bath. The thermostat tank held 50 or 60 litres of water, and was 

 kept stirred and in uniform temperature throughout, by having compressed 

 air bubbled through it from a number of jets distributed evenly over the 

 bottom of the tank. This method of stirring was very effective, for, when it 

 was in action, it was seldom possible to distinguish more than a hundredth of 

 a degree C. between any two points in the water of the tank. The sides of 

 the tank were protected externally by thick layers of felt, and its inner side 

 was surrounded by a coil of piping through which cold water could be 

 circulated, and the temperature of the tank kept constantly at 14 - 5° C. 

 The room in which the experiments were conducted was almost entirely 

 underground, and underwent little change of temperature between day and 

 night, or from one day to another, if the door was kept closed and the 

 windows protected. The experiments were carried out in the months of 

 July, August and September, when weather conditions were most favourable 

 for work of this kind. It is the special merit of the differential method that 

 external temperature conditions can be largely neglected, so long as both 

 flasks used in making the differential determinations are affected to the 

 same extent by all variations of external temperature. 



In order to get the eggs to fertilise and segment regularly in the flasks, it 

 was found necessary to carry out some form of aeration. To accomplish this, 

 air was slowly bubbled through the contents of both flasks at regular intervals 

 during an experiment. This also served to stir up the eggs and prevent their 

 settling in a dense mass in the bottom of the flasks. The air used in the 

 aeration and stirring process was first passed through a large wash-bottle, half- 

 filled with sea-water, sunk in the middle of the thermostat tank. The air 

 from this bottle was then led into each flask by fine rubber tubes, which 



VOL. xcill. — B. 2 G 



