That different species of animals have characteristically different heart rates is a well-known fact. The present investigation is a continuation of a study of some of the factors associated with the different pulsation rates observed in two of the common species of salamanders, namely Amblystoma punctatum and Amblystoma tigrinum.' I n a preliminary investigation ( '27), the heart rudiments were interchanged between embryos of A. punctatum and A. tigrinum, and it was found that the heteroplastically grafted organ maintained the inherent contraction rate of the donor species, although it became innervated by the host.
I n a more extensive study ('39b), additional data showed that the grafted heart underwent histological and functional differentiation at the rate of unoperated individuals of the donor species, although the graft did not grow at the donor rate, but gradually assumed the size characteristic of the organ of the normal host.2 When the heart rudiment was grafted from embryos of the larger animals, A. tigrinum, to embryos of the smaller animals, A. punctatum, the transplanted organ never grew as rapidly as it would have grown had it remained in situ. However, for a considerable time, it did grow faster than the normal heart of the recipient species. I n other words, regulation of the growth rate began early but was not completed until late in larval life when the animals were approaching metamorphosis. Conversely, when .the heart of A. punctatum was transplanted to A. tigrinurn it grew more slowly than the normal heart of the recipient species during the embry-* A study of the heart rates in these species was stimulated originally by Harrison's experiments ( '24), in which he demonstrated interesting differences in the growth rates of heteroplastically transplanted limbs.
In this respect, the heart grafts behaved Like the spinal cord grafts described by Detwiler ( '32) and differed from the limb and eye transplants reported by Twitty and Schwind ( '28).
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