PHYSIOLOGICAL EFFECTS OF LACK OF OXYGEN 405 



than that furnished by the assumption that the oxygen 

 "stimulates" the heart. 



I was especially interested in comparing the effects of 

 lack of oxygen on the beat of the heart in Ctenolabrus and 

 Fundulus embryos. Does the same difference in behavior 

 toward lack of oxygen exist here as in regard to cleavage ? 



The heart begins to beat and the circulation is estab- 

 lished in Ctenolabrus embryos as early as forty-eight hours 

 after fertilization. If such forty-eight-hour-old embryos, 

 which are still contained within the eggs, are introduced into a 

 gas-chamber through which a current of hydrogen is passed, 

 the heart usually comes to a standstill in from three to ten 

 minutes after the current of gas is turned on. The activity 

 of the heart does not, however, gradually fall to zero, but the 

 heart comes to a standstill suddenly when the number of 

 heart-beats has decreased but little or not at all. In one case 

 the heart beat about 90 times a minute before the hydrogen 

 was admitted. Hydrogen was then passed through the 

 gas-chamber, and after four minutes the heart still beat 89 

 times; two minutes later it beat 78 times, and in the following 

 minute 77 ; in the next minute the heart came to a sudden 

 standstill. After hydrogen had been passed through the 

 gas-chamber for only seven minutes, and when the number 

 of heart-beats had fallen only from 90 to 77 a slight de- 

 crease only the heart suddenly stood still; at that time 

 blood was still circulating beautifully. 



In a second experiment the number of the heart-beats 

 was 108 per minute at the beginning of the experiment. 

 Two minutes after turning on the hydrogen gas the heart 

 beat 105 times, and three minutes later 108 times a minute. 

 During the next minute the heart stood still after having 

 beaten 23 times in the first eighteen seconds of that minute. 

 The heart stood absolutely still for four minutes, after which 

 it gave a few weak pulsations. For the next three minutes 



