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PROFESSOR D. WATERSTON ON 
ventricle, but that fact is not sufficient in itself to justify the division of the right 
ventricle into two parts of separate developmental origin. The appearances strongly 
suggest that the mammalian right ventricle is formed entirely from a primitive 
chamber homologous with the bulbus cordis of fishes and reptiles. If the mammalian 
right ventricle is derived solely from the bulbus cordis, and if .the mammalian 
muscular interventricular septum be the bulbo-ventricular septum, then the muscular 
interventricular septum of the reptilian heart is not represented as such in the heart 
of mammals, and the anterior and posterior ventricles of the reptile are not homo- 
logous with the right and left ventricles of the human heart. 
Greil has pointed out also that in Varanidee there is no question of the division 
of the ventricle into a right and a left ventricle, for both of the atrio-ventricular 
openings belong to the dorsal chamber (the ventricles being dorsal and ventral, and 
not left and right). It must, however, be pointed out that the anterior ventricle of 
the reptilian heart, consisting as it does of bulbar and ventricular portions, separated 
partially from one another by a muscular ridge, gives rise not only to the pulmonary 
artery but also to the left systemic aorta, and hence it is not entirely comparable 
with the right ventricle of the mammalian heart. 
It can, moreover, be shown that in the development of the heart of man the 
bulbus is divisible into two portions — an upper cephalic and a lower caudal ; and that 
these two parts give rise to different parts of the adult right ventricle — the upper to 
the infundibulum, and the lower to the expanded portion of the ventricle. 
5. Developmental Changes in the Bulbus Cordis. 
If we assume, on the grounds previously stated, that the chamber in embryo Si 
which forms the right portion of the ventricle is the bulbus cordis, this specimen 
affords a convenient starting-point from which . to trace the subsequent changes in 
this part of the heart tube. 
In this specimen, as in other specimens of similar age described by other writers, 
that portion of the heart is flask-shaped, and is divisible into a caudal spherical and 
an oral cylindrical portion. In the latter the lumen is almost completely divided 
into two channels by the projection into the interior of the bulbar cushions, while 
in the former the bulbar cushions are widely separated from one' another and diverge 
on opposite sides of the ventricular cavity. 
Distal Portion. — Plate-fig. 4 shows a reconstruction of a portion of the distal 
part of the bulbus, with one of the two large cushions found in it. This cushion 
begins above at the separation of the root of the combined sixth aortic arches from 
the truncus arteriosus, and extends uninterruptedly downwards in a clockwise spiral. 
On the opposite wall, which has been removed, there was another cushion of 
similar extent. 
The upper portion of these cushions corresponds precisely to the “ distal bulbar 
swellings” 1 and 3 respectively figured by Tandler in the heart of an embryo 
