Temperature and Heart-Rate in Fundulus Embryos

Abstract

ABSTRACT On the 4th day after fertilisation, cardiac rhythm in Fundulus embryos, in all probability, is myogenic. Between 5 ° and 25 ° C., one of two temperature characteristics prevail and fluctuate either about μ = 16,300 or 14,300. On the 12th day, embryos, presumably neurogenic, expose prevailing increments of 20,900 or occasionally of 18,200. A definite critical region is localised about 20 ° where the prevailing increments break to the orders 16,000, 14,000 or 12,000. Over the corresponding range embryos about to escape from the egg-case, exhibit, in the only two instances recorded, μ = 23,000, 20,900 and 16,300. On presumable restoration of myogenic conditions in embryos of the 12th day, μ may be 14,300 over the entire range. The frequency of μ = 14,300 is greater than in normal larvae of the same age, whereas μ = 20,900 is restricted to the lower temperatures. In their bearing on the differences between myogenic and neurogenic rhythms, these results become explicable if we imagine an underlying mechanism identical for both cases. The same increments either occur or suggest themselves at all stages of development, or, if at times some appear to be distinctive of neurogenic Fundulus, these can be promptly duplicated from the list of increments shown by myogenic Limulus, e.g. 20,900. Clearly, however, the most frequent characteristics found belong to categories exposed by a great variety of biological acts, and their association in cardiac rhythms is common. These two facts harmonise well with the recent work on the metabolism of muscle and nerve.