SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 669 



wards. During this period 2000 eggs consumed between 82-086 and 

 98-00 mgm. of oxygen, and when this amount was compared with 

 the amount of solid matter used up during incubation, the results 

 were very close, for the eggs lost 78-3 mgm. of protein and the oxygen 

 used accounted for between 65-67 and 77-8 mgm. Further con- 

 sideration of the work of the Dakins will, however, be deferred to 

 the sections on General Metabolism, Energy Sources, etc. Burfield 

 investigated the respiration of plaice eggs with a view to finding 

 out whether the gaseous exchange fell off during a single experi- 

 ment in the closed chamber, and, if so, why. It is extremely tanta- 

 lising that he apparently made no record of the age or state of 

 development of his eggs, classing them simply as "young"; his 

 figures which might, therefore, have been useful, as showing dif- 

 ferences in respiratory exchange with age, have no value for our 

 main purpose. The fall in the rate of oxygen consumption of aquatic 

 organisms might be due, he argued, to a combination of four factors: 

 (a) handling of the animal at the beginning, (b) absorption of the 

 available oxygen, (c) accumulation of carbon dioxide or other excreta 

 and {d) feeding having gone on immediately before the experiment. 

 Factors {a) and (d) do not operate in the case of eggs, and factor {b) 

 was avoided by using a sufficient volume of sea water. Accordingly 

 Burfield found that the third possibiUty was very important, and 

 was able to depress considerably the rate of oxygen consumption of 

 the developing plaice eggs by adding small amounts of carbon 

 dioxide to the water. They were far more sensitive to this than to 

 reduced partial pressures of oxygen. Urea had no effect. If the 

 eggs were frequently moved so as to prevent accumulation of carbon 

 dioxide in their immediate vicinity no fall in oxygen consumption 

 was detectable, and the amount absorbed during the ist hour would 

 be a fair average of the values for the succeeding hours. Whitley 

 had already noted that the amount of variation from the normal pH 

 which plaice eggs will tolerate is very small indeed, and that a 

 disturbance of the equilibrium towards the acid side is much more 

 fatal than a disturbance towards the alkaline side. These facts fit 

 in together well, but Whitley's work was not altogether confirmed 

 by Hopkins, and the latter actually found that eggs of the trout and 

 perch would not develop properly in the absence of very small 

 amounts of free carbon dioxide. Burfield also occupied himself with 

 the respiratory quotient of the eggs, measuring the carbon dioxide 



