THE DEVELOPMENT OF THE HEART IN MAN. 
269 
(2) The upper cylindrical portion is now oblique in direction, and slopes from* 
right to left and towards the dorsum as it ascends. 
(3) The termination of the heart tube, i.e. the junction with the truncus arteriosus, 
can be clearly determined by the structure of the wall. 
The lumen of the truncus is divided into two separate channels, aorta and pulmonary 
artery, and each of them, is continuous with a portion of the incompletely divided 
lumen of the cylindrical portion of the bulbus cordis. 
The increasing obliquity of the region has led to a lengthening of the convex 
Text-fig. 6. — Transverse section (slide 10, section 10) of heart of embryo Ml. 
portion. The bulbar cushions are distinct, though only two can be made out (see 
also embryo Ml). 
Embryo 9 mm. in Length (Ml). 
Reconstruction at 100 Diameters. — The heart of this embryo resembles closely 
the former specimen (Bl) both in size and in shape. The external form is almost 
identical with that of Bl (Plate-fig. 5). There are the same wing-like expansion of 
the atria, the surface division between right and left ventricles, and the modified 
flask-like character of the right ventricle passing orally into a tubular portion. 
Internal Structure. — The relationship of sinus venosus to atrium resembles so 
closely that in Bl that it requires no further description. The disposition of the 
venous valves and septum primum is practically identical with the former specimen 
(text-figs. 4 and 6). 
Sections in this specimen stained with picro-indigo-carmine indicate very clearly 
the extent and connections of the muscle coat. 
Atrio-ventricular Junction (text-fig. 6). — The upper endocardial cushion, as before, 
lies against the septum primum dorsally, while ventrally its upper part forms the 
dorsal wall of the ventricle. 
