SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 



647 



explanation. The difficulties which have already been mentioned 

 with respect to Vies' work on the carbon dioxide production naturally 

 operated in Rapkine's work as well, though it was much more satis- 

 factory in so far as the gas was bubbled off, and the carbon dioxide 

 estimated by a baryta method. The alkali reserve, of course, remained 

 an unknown factor. The curve for carbon dioxide given out per hour 

 per gram dry weight followed for the first 9 hours a regular course 

 concave to the abscissa. After that time, however (see Fig. 114), 

 it dropped to a trough at the 15th, and afterwards rose again to 

 a peak at the 24th, after which it again fell and rose to the 40th 

 hour, i.e. to the pluteus stage. The large waves seen in the curve 





O 6 



2 4 6 8 10 12 14 re 18 20 22 24 26 28 30 32 34 36 38 40 

 Time in hours after fertilisation 

 Fig. 114. 



after the loth hour are undoubtedly due to the formation of the 

 skeleton, which is so prominent a feature of the later development 

 of the echinoderm egg As Rapkine & Prenant showed, very definite 

 pH changes take place in the blastocoele cavity as the calcareous 

 spicules are formed and the mesenchyme develops, which are 

 associated with the retention of carbon dioxide to form calcium and 

 magnesium carbonates. Moreover, as Herbst showed, if potassium 

 chloride is added to the water, skeleton formation is inhibited, and 

 in such a case Rapkine found a regularly rising carbon dioxide 

 curve. The respiration curves after the loth hour are therefore 

 difficult to interpret, and attention must rather be directed to the 

 time elapsing before that point of development is reached. During 

 this early period, the metabolic rate, both as regards oxygen and 

 carbon dioxide, is rising, but significantly not at the same rate, so 



