1883.] VALVES IN ECHIDNA AND OKNITHORHVNCHUS. 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 con- 

 nected 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 eft'ected, and first the membranous 

 condition of the flaps, then of the 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 teli of its 

 original condition. 



As Bernays has pointed out, the right cardiac valve of Ornitho- 

 rhync.hus corresponds to the third stage of the mammalian onto- 

 genetic 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 car- 

 diac valves of Ornithorhynchus (see my 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 Edndna is precisely that 

 described by Bernays as the commencement of his fourth stage. 

 Bernays figures (plate xxxii. fig. 6) an adult human heart, in which 

 one of the papillary muscles of the right ventricle has precisely that 

 direct attachment to the membranous part of the valve and de- 

 ficiency of chordae tendineae which I have described as character- 

 ising Echidna's right cardiac valve. Echidna is thus, when judged 

 by the series afforded by the facts of ontogenesis, distinctly inter- 

 mediate in this respect between OrnithorhyncMis and the higher 

 Mammalia. 



We may further inquire what light the ontogenesis of the mam- 

 malian heart throws upon the absence of the septal flap in the 

 Monotremata. It appears that the consideration of ontogenesis 

 enhances the importance of the distinction between Monotremes and 

 other mammals afforded by this charncter. According to Bernays 

 the Crocodile-heart is in the second stage of development. The 

 membrane of the large septal valve is not due to the degeneration of 

 a secondary muscular valve, but is the primary endocardial valve ; 



