METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 55 



eggs. Although the eggs were frequently washed, particularly in 

 experiment 4, this source of error was probably present. It 

 tends of course to make the oxygen consumption appear too great. 

 In experiment 4 in which the eggs were thoroughly washed twice 

 daily, the results are probably more reliable than in the other three 

 experiments ; and in this experiment no increase was observed in 

 later stages. 



These experiments lead us to believe that the rate of oxygen 

 consumption in the development of Funduhis is highest at the 

 time when the germ ring is in the neighborhood of the equator, 

 early on the second day of development. It is probably actually 

 highest per unit weight of protoplasm since from that time on 

 the amount of protoplasm increases greatly but the oxygen con- 

 sumption does not increase in like proportion ; in fact, a consider- 

 able part of the oxygen consumption after the third day is due 

 to the activity of the heart. As the embryo is continually increas- 

 ing in size after this time while the oxygen consumption shows 

 relatively little increase we may reasonably conclude that the 

 oxygen consumption of the embryo per unit weight is actually 

 decreasing. In other words, senescence is already in progress. 



The study of the carbon dioxide production yielded similar 

 results. The carbon-dioxide production per unit time increased 

 up to the early part of the second day of development after which 

 it fell, rising again in later periods. 



This result, that the metabolic activity of the embryo is at its 

 highest point at the period when the germ ring is near the 

 equator, is in harmony with and furnishes an explanation of 

 previously known facts. Child determined the susceptibility of 

 the eggs of Funduhis to phenyl urethane ('15^, p. 416). He 

 found that the embryos are killed more quickly at this stage than 

 at any other stage. Since susceptibility is, as I have pointed out 

 in the introduction, a measure of metabolic rate, this result of 

 itself shows that the metabolic rate is highest at that period. My 

 experiments confirm this result of Child's and further illustrate 

 the reliability of the susceptibility method as a measure of rate 

 of activity. Various investigators, as Stockard and Kellicott, 

 whose work is considered at greater length later, have noted that 



