THE VARIATIONS DURING LIFE 109 



When Bohr and Hasselbalch calculated the respiratory exchange 

 per kg. and hour for the later stages of the development (from the fifth 

 to the eighteenth day) they found that it is not very variable (590 to 

 830 c.c.), while the average, 718 c.c., is about the same as that found for 

 the grown hen in Regnault and Reiset's experiments. It must be borne 

 in mind, however, that the embryo is very quiet, while the experiments on 

 hens were not made under standard conditions. For the earlier stages 

 a much higher respiratory exchange per kg. and hour is calculated, but 

 here the determination of the amount of living tissue which takes part 

 in the metabolic processes is very uncertain and in all cases too low, 

 because the " yolk sac " could not be included. 



Material has not been furnished by these experiments which will 

 allow a definite distinction to be made between the true basal 

 metabolism of the embryo and that required for the activity of tissue 

 formation. In the opinion of the writer the experiments do show, 

 however, that the tissue-forming activity can (during the later stages of 

 development at least) be responsible for at most a small fraction of 

 the total respiratory exchange, since the formation of new tissues takes 

 place with decreasing velocity, while the respiratory exchange is during 

 the whole period from the ninth to the eighteenth day practically pro- 

 portional to the weight of the embryo. 



Hasselbalch [1902] has attempted to show that during the first 

 hours of incubation free oxygen is not only not absorbed but actually 

 produced by the eggs. His results are due, however, to the large 

 amounts of gas (nitrogen and oxygen) which at ordinary temperatures 

 are absorbed by the fatty substances in the yolk and partially liberated 

 when the eggs are incubated at 38. 



Bohr [1900] has studied the respiratory exchange of mammal 

 embryos by the following elegant method. Pregnant guinea-pigs or 

 rabbits were narcotized with urethane, a tracheal cannula was intro- 

 duced, and the pulmonary gas exchange measured in lo-minute 

 periods. The uterus was laid open by bloodless operation, and the 

 respiratory exchange of the embryos could now be suspended either 

 for a time or permanently by compressing or ligating the umbilical 

 cords. Comparisons were made between normal periods in which the 

 circulation of the embryos was not interfered with and such in which 

 it was suspended. The results of five series of experiments on guinea- 

 pigs were as follows : 



