SECT. 6] OF THE EMBRYO 847 



selective absorption of the carbon dioxide produced by the organism 

 as a whole by the mesenchyme cells for the manufacture of calcium 

 carbonate, a process which might well be expected to make the liquid 

 of the blastocoele cavity more alkahne than usual. Next Bouxin found 

 that a fall of pH irrespective of the acid employed would produce in 

 Strongylocentrotus lividus larvae a retardation of skeleton formation or a 

 complete stoppage, or even if the p¥L was lowered below 6-4 a re- 

 gression of spicules already formed. Rapkine & Bouxin accordingly 

 micro-injected indicators into the blastocoele cavity at different ex- 

 ternal hydrogen ion concentrations, and in fact found that when the 

 exterior /?H went down to 6 that 

 of the blastocoele cavity also 

 went down, and almost as far, 

 keeping above it, however, to the 

 extent of two or three tenths of ^^h 

 a />H unit. In the extreme cases 

 there seemed, then, to be an 

 actual solution of the spicule. 

 Rapkine & Bouxin found that 

 during this process the j&H of the 

 liquid in the blastocoele cavity 

 was maintained rather stable at ^^' ^° " 



/>H 6*25, and they suggested that the regression of the skeleton might 

 to a certain extent be considered as a defence mechanism in view of 

 the fact that death invariably occurred when the blastocoele pYi had 

 reached 6-o. In the case of younger embryos, where the mesenchyme 

 cells had just appeared, a small fall in external />H causes a retardation 

 or an inhibition of skeleton formation, while the j&H remains at about 

 7-4; thus the mesenchyme cells are removing carbon dioxide, but the 

 external acidity prevents the j&H rising. 



Contributory evidence showing the retention of carbon dioxide 

 during spicule formation was provided by Vies & Gex, who placed 

 developing echinoderm eggs in a small chamber surrounded by a 

 dilute indicator, and then examined the system from time to time 

 spectrophotometrically. They obtained the figure shown in Fig. 209, 

 from which it can be seen that during the first 4 hours the />H fell 

 steadily (N.B. no sign of rhythmic change), but that between the 

 4th and the 6th hour there was a kink in the curve, indicating a 

 retention of carbon dioxide, and even a slight absorption of it. This 



70 80 90 100 no 120 

 hours 



