746 THE RESPIRATION AND [pt. iii 



dictions of them, such as Wesselkin in 191 3, who worked with chick 

 embryos, and found that an atmosphere of 5 per cent, oxygen killed 

 them in 48 hours, but that 10-15 per cent, permitted continued de- 

 velopment for 72 hours. It may be concluded that, although there is 

 no strong reason for believing that embryonic development can ever go 

 on anaerobically, there are yet some curious facts which ought to be 

 looked into before a final decision is reached. And the work of Reiss 

 & Vellinger referred to on p. 869 suggests that the energy required for 

 cleavage may be obtainable without free oxygen, by electron transfer. 



4-18. Metabolic Rate in Embryonic Life 



Attention must now be directed for the last time to the metabolic 

 rate question. We have already seen that Gayda (for the toad). Wood 

 (for the pig) , Dubois (for the human being) and Ginglinger & Kayser 

 (for the pigeon and the rabbit) have shown that the respiratory rate 

 has a point of maximum intensity during the life-span, though sub- 

 sidiary kinks on the curve may exist (as at puberty). It is extremely 

 probable that, in the rising rate curves of echinoderms and amphibia 

 early in development, we see the ascending part of a curve, and, in 

 the falling rates of avian embryos, we see the descending part of the 

 same curve. What factors determine the point in the life of the 

 individual at which the peak shall occur are as yet obscure although, 

 as we have seen, Ginglinger & Kayser explain it by the onset of 

 heat-regulation . 



It must first be pointed out that the falling part of the curve has 

 often been plotted, and is well seen in the measurements of Magnus- 

 Levy and Falk on man ; of L. Mayer on the hen, the duck, and the 

 guinea-pig ; of Sayle on dragonfly larvae ; of Krarup on rabbits and 

 of Benedict & Riddle on pigeons^. Mayer noticed that, in the case 

 of the guinea-pig, the fall after birth followed the course of a regular 

 hyperbola. The following figures, again, exemplify it. 



C.c. carbon dioxide 

 put out per kilo 

 per hour 

 Very small rabbit embryo (Bohr) 750 



Rabbit embryo nearly at term (Bohr) 509 



Adult rabbit (Krarup) ... ... 450 



Its extremely general character is shown by the fact that Hee 

 obtained a curve for intensity of gaseous exchange during the develop- 



^ Other instances are cows (Brody) ; cladocerans (Obreshkove) ; molluscs (Hopkins) . 



