622 THE RESPIRATION AND [pt. iii 



"The amount of carbonic acid gas", said Pott & Preyer, "given 

 off by a developing embryo in a six-hour period, was four times as 

 great at the beginning of the third week as it had been at the begin- 

 ning of the second week, and on the twentieth day it was nearly 

 ten times as great as at the end of the first week. During the course 

 of the second week the carbonic acid gas exhaled is more than 

 doubled and during the course of the third week it is more than 

 doubled again." Pott & Preyer rightly felt it to be a very important 

 finding that the embryo used up oxygen and gave off carbon dioxide 

 long before the establishment of a pulmonary mechanism. This work 

 would seem to be of importance in the history of the progress of 

 the conception of tissue respiration, but no one has yet accorded 

 such a credit to it. The absolute values which Pott & Preyer 

 obtained for the amounts of oxygen and carbon dioxide concerned, 

 resemble fairly closely the figures of later workers. By performing 

 a simple calculation 



where K^ and K^ is the loss of carbon dioxide by fertile and infertile 

 eggs respectively, W^ and Wy_ the loss of water by fertile and infertile 

 eggs respectively, and Gg and G^ the loss of total weight by fertile 

 and infertile eggs respectively, Pott & Preyer calculated the amounts 

 of oxygen actually used up by the embryo each day. They did not 

 make any direct estimations of oxygen. One of their conclusions was 

 not supported by later experiments, for they said that the loss of 

 water was markedly affected by the presence of the embryo, affirming 

 that fertile eggs lost much less per day than infertile ones. This 

 statement cannot now be accepted without modification. 



Although the greater part of our knowledge of the respiration of 

 the mammalian embryo is derived from researches undertaken at a 

 comparatively early period in the last century, the consideration 

 of it will be deferred until the discussion of that subject. It need only 

 be said here that Girtanner and Schehl in 1 794 seem to have been the 

 first workers to say definitely that the mammalian foetus "asphyxiates 

 itself if it does not receive oxygen from the blood of the mother". 

 Experiments by Scheel in 1798 led to the same conclusion. Runge 

 & Schmidtt and Zweifel observed the presence of oxyhaemoglobin 

 in the umbilical vessels. 



Before proceeding to the investigations of more recent times, we 



