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PROFESSOR D. WATERSTON ON 
the cushion tissue between the ostia, and ventrally by the concave free margin of the 
interventricular septum (text-figs. 11 and 12). 
The boundaries are rendered more complex by the approximation and extensions 
of the bulbar septum. The proximal edge of this septum is concave, and the ventral 
right end is continued on to the ventral and upper part of the interventricular septum ; 
while the other end, dorsal and to the right, joins with the tissue between the venous 
ostia and is continued on to the interventricular septum. From its right margin a 
prolongation of this septum of the bulb extends towards the right side above the right 
venous ostium, and joins the flap which sweeps on to the interventricular septum. 
The interventricular opening is, in this specimen, unobscured by valve cusps on the 
septal wall, and its boundaries are shown with remarkable clearness in the model. 
Text-fig. 12. — Linear reconstruction of heart of embryo S4, 
Tubular • Portion . — There is a complete separation of the aorta from the infun- 
dibulum of the right ventricle. 
There is a considerable change, however, in the general position and diameter of 
the infundibulum of the right ventricle from the condition found in the earlier stages. 
Comparison of the linear reconstructions of these specimens shows the changes. 
The lumen within the tubular portion of the bulbus cordis has expanded and now 
forms a chamber continuous with the cavity of the lower part of the right ventricle. 
Embryo (16 mm. in Length), Prof. Peter Thompson’s Specimen. 
Plate-figs. 11 and 12. 
In this specimen, and in all the later ones, a definite advance in the development 
of the heart is marked by the closure of the interventricular orifice. It was pointed 
