Abstract
For a number of Limulus embryos the relationship has been determined between temperature and the frequency of cardiac contractions, during stages of development when the (myogenic) heart is still devoid of a nervous system. A thermostat maintaining any desired temperature between 0.05° and 45° to within ±0.01° or better, and quickly adjustable, was devised to facilitate this work. The temperature characteristics obtained are not the same in all individuals. The values μ= 11,500 and 16,400 are usual; but μ = 20,000 and μ = 25,100 also occur. These are well recognized values in numerous other vital processes, but differ from the magnitude (μ = 12,200) which seems typical for the thermal control of frequency of contraction in the heart of the adult (the temperature of the ganglion being altered). This need not be interpreted as corresponding to an essential difference between myogenic and neurogenic rhythm. The critical temperatures for heart rhythm also differ among individuals; the following are of definite occurrence, in different cases: 9°, 20°, 27°, 31°, 41°, 45°.
The results are chiefly important as indicating relatively greater metabolic lability in embryonic than in adult tissues. It is concluded that the apparent inconstancy of μ for contraction rate in explants of chick myocardium (H. Murray) is due to the presence of several functional pacemakers having different temperature characteristics, corresponding to the finding of different μ's for the hearts of similar Limulus embryos. This is proved by the fact that in cases giving atypical μ's, the latitude of variation is not a constant fraction of the mean; the marginal heart-beat frequencies yield typical μ's.
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