288 
PROFESSOR D. WATERSTON ON 
ventricles becomes shallower, especially dorsally, and the dorsal half of the tubular 
portion of the right ventricle becomes directly continuous with the left ventricle. 
The ventral surface of the right ventricle shows a groove at the junction of the 
two parts forming it, but this groove gradually becomes shallower and disappears by 
the 12'5-mm. stage. 
The proximal portion of the right ventricle retains a triangular form ; the distal 
tubular portion becomes more and more oblique. The groove on the surface of the 
conus arteriosus which marks the septum dividing that vessel into the pulmonary 
artery and the aorta, becomes deeper and is prolonged spirally on to the bulbus 
cordis, marking off a dorsal from a ventral portion. The dorsal portion becomes 
reduced in length, and its root passes to the left side and joins the base of the left 
ventricle. The ventral portion, at first tubular, dilates to a marked extent and 
forms the infundibulum of the adult right ventricle. 
Dorsally the sinus venosus and the atrium become largely incorporate at the 
9-mm. stage. The transverse portion of the sinus venosus become applied to the 
dorsal surface of the heart and lies obliquely on the dorsal surface of the atrium. 
The dorsal wall of the atrium expands with the formation of the pulmonary veins, 
and the sinus venosus (transverse portion) is displaced and comes to lie in the atrio- 
ventricular groove, or circular sulcus of the adult heart. 
The adult arrangement of four pulmonary veins has been reached in the 28'6-mm. 
embryo. 
2. Developmental Changes affecting the Junction of the Sinus Venosus 
and Atrium. 
The portion of the sinus venosus which participates in these changes is the right 
horn, and to a lesser degree the transverse portion. 
The changes which occur in this part of the heart are known to consist largely in 
the incorporation of the right horn into the atrium, so that these two portions come 
to form in the adult a single chamber in which only slight demarcation can be 
determined of the sinus portion from the atrium proper. 
Stated briefly, the changes involved in this incorporation are as follows : — 
At the junction of the sinus and atrium there is externally a slight groove, 
a narrow smaller chamber joining on to one of larger size. The groove becomes 
invaginated at each side, producing in the interior two loose flaps of tissue — the right 
and left venous valves, the right one being formed before the left. Whether these 
valves are formed merely by a passive invagination or by active ingrowth cannot be 
determined. In histological structure, however, the right venous valve, and to a less 
degree the left one, differ from the wall of the sinus or atrium ; the cells are larger 
and less fibrillated, and numerous vacuoles are present. 
The sinus opening into the atrium resembles the “ valvula coli ” of human anatomy 
in being merely a slit on the summit of a prominent ridge, and the ridge is prolonged 
