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PROFESSOR D. WATERSTON ON 
but still tubular part ; and, finally, the lower part of the cavity of the ventricle with 
the valve cusps, papillary muscles, and trabeculae of the ventricular cavity proper. 
Heart of 22-mm. Embryo (B3). 
Both ventricles present a rounded outline ; the wall of the left ventricle is almost 
twice as thick as that of the right ; both show extensive trabeculae ; the cusps of the 
aortic valve are well formed and plump ; and the interventricular septum is completely 
formed, though extremely thin in the membranous part. 
The right atrio-ventricular orifice, widely open, possesses right and left cusps, both 
of which, especially the right, are undermined by trabecular musculature, and their 
apices are continued by stout muscle bundles to the ventricular wall. A wedge or 
cone of rudimentary connective tissue penetrates for a short distance into this 
Text-fig. 13. — Transverse section (slide 26, section 23) of heart of embryo B3. x 20. 
marginal cusp, and on the atrial surface of the cusp an oval area of cushion tissue is 
present. The septal cusp shows a condition similar to those seen in S3. 
Left Atrio-ventricular Orifice. — Shows very clearly the origin of the anterior 
cusp of the mitral valve from the endocardial cushion tissue of the atrio-ventricular 
junction, and as yet shows no muscular tissue in its substance ; while the posterior or 
marginal cusp is distinctly muscular in structure. 
Atrio-ventricular Bundle. — Rudiments of this are very clearly seen within the 
loose endocardial tissue of the atrio-ventricular junction (text-fig. 13). 
The condition of the atria is, on the whole, similar to that shown in S3. 
30-mm. Embryo, Prof. Bryce’s Specimens. Plate-figs. 14 and 15. 
The anterior view of the model of this heart, reconstructed 50 diameters, is shown 
in Plate-fig. 14. 
External Form. — The external form of the heart shows it to have advanced 
considerably towards the adult form, and the internal structure of the ventricle is 
practically the same as is found in the adult, the right and left ventricles being 
completely separated. The atria, however, communicate freely with one another. 
