SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 719 



Two attempts have been made to calculate calorific quotients for 

 the chick embryo since the publication of exact data for the heat 

 production and the oxygen consumption. The results of my attempt 

 to do this are shown in Fig. 157, where calorific quotient is shown 

 plotted against the age (a) using Bohr & Hasselbalch for heat and 

 Hasselbalch for oxygen, and (b) using Bohr & Hasselbalch for heat 

 and Murray for oxygen. In both cases there is a fall to the 9th day 

 followed by a rise lasting approximately for the rest of the incubation 

 period, but a glance shows that the values are grossly removed from 

 the theoretical. The dotted line drawn between the horizontal lines 

 shows the course that would theoretically be taken by the calorific 

 quotient supposing that carbohydrate was first burned, then protein, 

 and finally fat. As can be seen, the experimental curves do more 

 or less follow that rhythm, but little weight can be attached to 

 such a correspondence in view of the kink on the heat-production 

 curve (see Fig. 146), which almost certainly is responsible for 

 the drop in the calorific quotient at the 9th day. Nothing would 

 be more welcome than a redetermination of the heat-production 

 curve of the chick embryo, for we should not only then have a better 

 idea whether the kink in question is real or not, but also with im- 

 proved calorimetry less heat would be lost and the calorific quotient 

 would probably fall within its proper limits. Cahn's views on these 

 questions are discussed in the section on the Energetics and Energy 

 Sources of the embryo. 



4-14. The Air-space and the Shell 



No mention has so far been made of the recent work on the air- 

 space, a structure present in many kinds of eggs, but occupying a 

 specially prominent position in the case of the chick. Its origin in 

 birds' eggs is obscure, but, according to Lataste, no air-space is 

 present before laying, and its appearance is only due to the con- 

 traction of the egg-contents from the rigid shell as the egg cools after 

 leaving the parent body, Lataste supports this view by adducing 

 the fact that eggs with flexible coverings never have air-spaces 

 (e.g. lizards and serpents), and, in the case of shell-less birds' eggs, 

 which are sometimes laid, no air-space appears, though the envelopes 

 may be a little wrinkled. Indeed, as long ago as 1847, Goste had 

 observed that if a laying hen was killed, the oviduct ligated, 

 removed, and then immersed in a dish of oil, an oil-space formed 



