THE DEVELOPMENT OF THE HEART IN MAN. 
271 
The whole extent of the cushion tissue is uniform, and there is no indication 
whatsoever of “ distal” as contrasted with “ proximal” bulbar cushions. 
Embryo B2 (13 mm. in Length). 
In this heart the condition of the upper part of the bulbus cordis resembles, on 
the whole, the condition found in Bl and Ml. The pulmonary artery and the aorta 
are separated from one another in the upper part of their extent ; proximally the 
stems of these two vessels unite and are continued as a single stem into the upper 
part of the right ventricle. The bulbar cushions A and B are prolonged downwards 
on to the lateral ventricle wall and to the septum respectively. 
A trio -ventricular Junction . — The right bulbar cushion (B) joins with the right 
margin of the upper endocardial cushion above the right atrio-ventricular orifice. 
This orifice is of some size, and its lower boundary is formed by a tubercle of the 
lower endocardial cushion. The upper endocardial cushion extends to the base of 
the septum primum atriorum and passes across to the left side, forming the upper 
boundary of the left atrio-ventricular orifice. The lower endocardial cushion is 
prolonged to the apex of the interventricular septum, below the level of the inter- 
ventricular foramen, and forms a large mass extending from the dorsal wall of the 
atrium across the floor to the interventricular septum. From its left extremity a 
tubercle projects into the cavity of the left ventricle and forms the right margin 
of the left atrio-ventricular orifice. 
A small accessory left endocardial cushion forms the left boundary of that opening. 
The muscle wall of the ventricles shows a complex network of trabeculae, and a 
cortical layer which is considerably thicker in the left than in the right ventricle. 
In this specimen the endocardial cushions are very distinct, as the nuclei alone are 
stained, while the muscle wall is of a pinkish colour. 
The atrio-ventricular valve apparatus is rudimentary, but the stage of develop- 
ment is only slightly more advanced than in Bl. 
In other details the heart in this specimen resembles closely the heart in S4 ; 
and as the latter shows more perfect histological detail, a fuller account need not be 
given of B2. 
Heart of Embryo S4 (12’5 mm; in Length). 
The heart of this embryo was modelled at a magnification of 100 diameters, from 
sections 10 m in thickness. 
External Form . — The figure (Plate-fig. 9) shows the external form from the front. 
The atria, greatly expanded, enclose the upper segment of the bulbus cordis in a deep 
groove, and this portion of the bulbus lies almost horizontally. 
The ventricular portion resembles generally that of the earlier specimens. The 
left ventricle, somewhat spherical in shape, is separated from the right ventricle by 
a deep interventricular sulcus and by a notch on the caudal margin. 
