DEVELOPMENT OF FISH EMBRYOS 299 



days-old embryo comes to a standstill, and the embryo dies 

 immediately (that is, in less than an hour), in a 1.5 per cent. 

 KC1 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 KC1 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. KC1 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. KCi 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 



