the Echinoderm Egg during Fertilisation. 



215 



In the type of manometer employed in the following experiments the 

 instrument, when used for ordinary hlood-gas measurements is designed to 

 allow a small quantity of potassium ferricyanide, held in a small tube in the 

 stopper of the manometer chamber, to run down and mix with the blood 

 when the chamber itself is slightly rotated, so that a groove in the chamber 

 wall comes to lie opposite the opening of the small tube in the chamber 

 stopper. The instrument was readily adapted to the purpose of the present 

 experiments by replacing the ferricyanide by a drop of fresh sperm. The 

 eggs could then be fertilised within the closed chamber of the apparatus, 

 when both eggs and apparatus were in complete equilibrium with the 

 temperature of the water-bath in which the manometer chambers were 

 submerged. 



The calibration of the manometers for oxygen was carried out by the 

 Hoffmann (5) method, and for CO2 by the liberation in the chamber under 

 calibration of a known volume of this gas from 2 c.c. of a Na 2 C03 solution. 

 All calibrations were carried out under the conditions holding for the follow- 

 ing experiments. The temperature of the water of the thermostat in which 

 the chambers of the manometer were submerged was kept constantly at 

 14 - 5° C. by means of a coil of piping inside the bath, through which cold 

 water was kept circulating. This temperature is almost the same as that at 

 which the eggs are normally fertilised in the sea. Moreover, it is convenient, 

 as the solubility of CO2 in sea-water at this temperature and standard 

 barometric pressure is almost 1, so that the CO2 will distribute itself evenly 

 throughout both sea-water and the air space of the apparatus chambers, and 

 no correction need be introduced for solubility of the CO2 in the sea-water on 

 this account. 



Warburg (4), in his measurement of the C0 2 , has taken elaborate means to 

 determine the actual amount of CO2 given off, by using bicarbonate-free sea- 

 water and estimating the total alkali reserve of his eggs ; it is probable that 

 the use of bicarbonate-free sea-water in itself introduces conditions that 

 render respiration far from normal. 



In a number of determinations made on the solubility of C0 2 in sea-water 

 at the pressures obtaining under the conditions holding in the chambers of 

 the manometer in the following experiments, it was found this solubility 

 could be neglected, as it was so small in each case as to be well within the 

 error of the manometer reading.* It is only at tensions of 0'4 mm. pressure, 



* The change of pressure in the chamber of the apparatus during an experiment was 

 very small, being something of the order of 0'005 of an atmospheric pressure. The effect 

 of this on the solubility of carbon dioxide in the sea-water of the chamber could safely 

 be neglected. 



