PHYSIOLOGICAL EFFects oF Lack oF OxyGEen 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 introducedintoa 
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 
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