DEVELOPMENT OF FIsH EmBrRyos 299 
days-old embryo comes to a standstill, and the embryo dies 
immediately (that is, in less than an hour), ina 1.5 per cent. 
KCI solution, while an embryo of the same age which has 
been kept in this solution from the beginning continues to 
live in it, and may even show slight evidences of a heart- 
beat? To say that the embryo adapts itself or becomes 
accustomed to the poison gives us no new view of the ques- 
tion. Might it not be possible that the KCl is the more 
poisonous the greater the work done by the heart in the 
unit of time, and in consequence the greater the chemical 
changes going on in it? 
According to this, it would be intelligible why a normal 
embryo, when put into a 1.5 per cent. KCl solution, dies 
within a short time, while an equally old embryo which has 
grown up in the poisonous solution is alive at the same time, 
and can even show evidences of a heart-beat. The heart of 
the former beats strongly, while that of the latter works only 
faintly—so faintly, indeed, that the blood does not even 
circulate. The embryo can live in a 0.5 per cent. KCl solu- 
tion as long as no great demand is made upon the activity of 
the heart. As soon as the heart begins to beat more strongly 
at the time of maturity, the embryo dies. This relation of 
the toxicity of the potassium to the development of energy 
in the protoplasm, or rather to the chemical changes deter- 
mining this development of energy, would hold not only for 
the heart, but also for all the other tissues. The entire ques- 
tion could be decided experimentally, if this has not already 
been done. 
6. All the remaining organs, especially the brain, eyes, 
ears, and mesoblastic somites develop in the Fundulus embryo 
without a circulation, without apparent anomalies. Only in 
one place, where no one has thus far suspected it, did a depend- 
ence on the circulation show itself in an unexpected way— 
in the marking of the yolk-sac, and possibly (but I wish to 
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