58 PUPATION OF TURTLE'S HEART. 



it, but not so powerfully, and even distilled water or MgCl 2 will often 

 give rise to a few final, weak pulsations. In other words, the heart 

 responds to any osmotic change, be it beneficial or injurious. It is 

 worthy of note, however, that if the heart ceases to beat in NaCl + 

 MgCl 2 it is usually impossible to revive it, even by CaCl 2 . 



The heart of the loggerhead turtle embryo, 14 days old, pulsates 

 more rapidly, and usually longer, in 0.7 per cent NaCl +0.03 per 

 cent KC1 than it does in 0.7 per cent NaCl. Thus the addition of a 

 small amount of KC1 acts as a stimulus. Physiologists are in dispute 

 concerning the action of potassium upon the " vertebrate heart," the 

 general opinion being that potassium depresses the heart. The litera- 

 ture of this subject is reviewed by Carlson (1906, Amer. Journ. Phys- 

 iology, vol. 16, p. 397). Much of the discrepancy in results arises 

 from the sweeping conclusions which physiologists have drawn in 

 applying to all vertebrates the results achieved from experiments 

 upon a few forms. Moreover, in some papers experiments are con- 

 ducted upon each salt separately, and the assumption is made that 

 the effect of a mixture of these salts is merely the summation of their 

 individual effects. Nothing could be more erroneous. For example, 

 calcium alone never stimulates, but even inhibits pulsation in Cassi- 

 opea, but in connection with sodium and potassium chlorides it forms a 

 most powerful stimulant. 



In closing we will state that the heart of the embryo loggerhead tur- 

 tle behaves quite differently from that of the animal after hatching, but 

 we will leave the discussion of this and other points to a future paper, 

 wherein we hope to treat of the general effects of different salts upon 

 the hearts of various vertebrates and invertebrates. 



In conclusion it may be said that rhythmical pulsation can be sus- 

 tained only when an external stimulant is counteracted by an inhibi- 

 tor, so that the pulsating organism is in a state bordering upon the 

 threshold of stimulation. This allows the weakest internal stimuli to 

 produce periodic contractions. Each contraction either produces a 

 chemical change which periodically reduces the internal stimulus, or 

 the tissue can not again respond to the ever-present, constant stimulus 

 until after a period of rest. 



