THE DEVELOPMENT OF THE HEART IN MAN. 
295 
Relation of the Conus Arteriosus to the Chambers in the Adult Heart. 
From the condition shown in Thompson’s model (2), in which the bulbo- 
ventricular groove lies horizontally, through the specimen 2Wl and onwards to the 
stage of the 5‘2-mm. embryo figured by Tandler, the bulbo-ventricular groove is seen 
to rotate gradually from a horizontal to an oblique, and finally to an almost vertical 
position. In the last specimen mentioned the groove forms a practically vertical 
furrow on the ventral surface of the heart, extending from the left side of the base 
of the truncus arteriosus to the caudal margin of the ventricle, where a notch is 
found on this border of the heart. Such a notch is present at earlier stages also, 
and is present even in the model prepared by Thompson, though not shown in 
the figures. 
Born’s models of this region of the heart in the rabbit are not at all conclusive. 
In two hearts closely resembling one another, he shows in the younger a bulbo- 
ventricular cleft running obliquely on the heart wall ; and in a slightly older specimen 
a similar furrow is present, from which a depression extends across to the right side, 
and the latter is taken to be the bulbo-ventricular furrow, while the former is 
termed the interventricular furrow. 
He states repeatedly, however, that there was no clear indication of the division 
of the bulbus from the ventricle. 
Of my own specimens, the heart of the embryo of 6-mm. length follows closely 
upon the Tandler specimen from a 5'2-mm. embryo, so far as the external form is 
concerned. In it the figure shows a vertical furrow extending on the anterior 
surface of the heart to the caudal border. It corresponds internally to the inter- 
ventricular septum, and the appearances suggest that it is no more than a deepened 
bulbo-ventricular furrow. 
It must further be noted that in the lower part of the interventricular septum 
(septum ventriculorum inferius) there is to be found the termination of one of the 
proximal bulbar cushions. This fact seems to afford strong support for the view 
that this portion of the interventricular septum is derived from the bulbo-ventricular 
ridge ; for were the interventricular septum formed to the left side of the bulbo- 
ventricular septum, and within the cavity of the primitive ventricle, it is difficult to 
see how the bulbar cushion could be prolonged on to it. 
From the 6-mm. stage onwards through the 7- and 8-mm. and on to the 12-mm. 
specimens, the evidence from the study of the position of the bulbar cushions and the 
muscle wall goes to show that the bulbo-ventricular groove is the same as the 
interventricular groove of later stages. 
The right atrio-ventricular orifice is enclosed on its right side by one of the 
bulbar cushions, and the septal wall of the ventricle supports the other proximal 
bulbar cushion. 
It is true that neither of these cushions reaches to the very apex of the right 
