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Elsewhere two of the present authors (Davies and Francis, 1941) have expressed the opinion that a specialised cardiac conducting system, consisting of nodal and Purkinje fibres, is a newly evolved one in birds and mammals. The general morphology and topography of this system in the heart of the bird (Davies, 1930 a, 1930 b) is similar to that in the heart of the eutherian mammal, and we have postulated (Davies and Francis, 1941) that the system has undergone parallel evolution in these two classes of homoiothermal vertebrates in response to functional requirements, and that in particular its presence can be correlated with the rapidity of the heart-rate in proportion to its size. On the other hand, Keith and Flack (1907), Keith and Mackenzie (1910), and Mackenzie (1913) maintain that the sinu-atrial node, atrio-ventricular node, and atrio-ventricular bundle of the mammalian heart are remnants of more extensive tissues of similar structure in the hearts of lower vertebrates. They trace the evolution of the conducting system of the mammalian heart from a simpler and more definite form which they described in the fish, and state that as one ascends the animal scale the concentration and reduction of nodal tissue becomes more marked. We ourselves (Davies and Francis, 1941) have failed to find any nodal or Purkinje fibres in any part of the hearts of the following cold-blooded vertebrates: fish (eel, gurnard), amphibia (salamander, frog), and reptiles (tortoise, Mississippi alligator, Sphenodon). We therefore believe that, whereas the ordinary cardiac muscle is able to conduct the impulse for cardiac contraction quickly enough in these slowly contracting cold-blooded vertebrate hearts, a specially developed system is evolved in the much more rapidly beating hearts of birds and mammals. Amongst mammals themselves, the nodal and Purkinje fibres of the monotreme heart (Davies, 1931) have the same general disposition as those in the eutherian mammal. The present work is concerned with the conducting system of the heart of the remaining sub-class of mammalia, namely the Didelphia. No previous detailed study of this system in the marsupial heart has been made.

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Elsewhere two of the present authors (Davies and Francis, 1941) have expressed theopinion that a specialised cardiac conducting system, consisting of nodal and Purkinje fibres, is a newly evolved one in birds and mammals. The general morphology and topography of this system in the heart of the bird (Davies, 1930 a, 1930 b) is similar to that in the heart of the eutherian mammal, and we have postulated (Davies and Francis, 1941) that the system has undergone parallel evolution in these two classes of homoiothermal vertebrates in response to functional requirements, and that in particular its presence can be correlated with the rapidity of the heart-rate in proportion to its size. On the other hand, Keith and Flack (1907), Keith and Mackenzie (1910), and Mackenzie (1913) maintain that the sinu-atrial node, atrio-ventricular node, and atrio-ventricular bundle of the mammalian heart are remnants of more extensive tissues of similar structure in the hearts of lower vertebrates. They trace the evolution of the conducting system of the mammalian heart from a simpler and more definite form which they described in the fish, and state that as one ascends the animal scale the concentration and reduction of nodal tissue becomes more marked.