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Show 1883.] VALVES IN ECHIDNA. AND ORNITHORHYNCHUS. 13 rhynchus presents, in the adult, a condition which is common to all Mammalia as a transient phase of embryonic development. Dr. Bernays distinguishes four stages in the ontogeny (actual development) of the mammalian atrio-ventricular valves. In stage no. 1 he finds valve-like processes of the wall of the heart which are simply projections of the endocardium, and have nothing to do with the ventricular musculature. Such valves are comparable to the watch-pocket valves of the Fish and Amphibian heart. In stage no. 2 a relation is established between these endocardial processes and the musculature of the heart by the growth of muscular bands on their under surface. In stage no. 3 the muscular bands connected with the endocardial processes attain a relatively very great size, and effectively constitute the valve, the original endocardial processes becoming unimportant by their relative diminution in size; thus a secondary atrio-ventricular valve of muscular composition arises. In stage no. 4 the degeneration of the muscular tissue and its replacement by membrane is effected, and first the membranous condition of the flaps, then of tbe chordae connecting the flaps with the remnant of the muscular tissue now known as papillary muscles, is brought about; thus the purely muscular secondary valve becomes membranous, whilst only the papillary muscles are left to tell of its original condition. As Bernays has pointed out, the right cardiac valve of Ornithorhynchus corresponds to the third stage of the mammalian ontogenetic development, whilst the left cardiac valve of that animal corresponds to the commencement of the fourth stage, in which the muscular tissue has disappeared from the upper portion of the valve, but the attached portion of the papillary muscles has not yet broken up into chordae tendineae. Whilst confirming this distinction between the right and left cardiac valves of Ornithorhynchus (see m y former paper for a figure of the left cardiac valve), I would further emphasize the fact that the condition of the right cardiac valve in Echidna is precisely that described by Bernays as the commencement of bis fourth stage. Bernays figures (plate xxxii. fig. 6) an adult human heart, in which one of the papillary muscles of tbe right ventricle has precisely that direct attachment to tbe membranous part of the valve and deficiency of chordae tendineae which I have described as characterising Echidna's right cardiac valve. Echidna is thus, when judged by the series afforded by the facts of ontogenesis, distinctly intermediate in this respect between Ornithorhynchus and the higher Mammalia. W e may further inquire what light the ontogenesis of the mammalian heart throws upon the absence of the septal flap in the Monotremata. It appears that tbe consideration of ontogenesis enhances the importance of the distinction between Monotremes and other mammals afforded by this character. According to Bernays the Crocodile-heart is in the second stage of development. The membrane of the large septal valve is not due to tbe degeneration of a secondary muscular valve, but is tbe primary endocardial valve |
