122 
JANE I. ROBERTSON. 
in the elasmobranch it is the left half of the venous ring^ 
circling the gut, not the right, that disappears. 
Sinus Venosus. — The sinus venosus is formed by the 
junction of the ducts of Cuvier, and comes early to lie on the 
right of the auricle, opening on the right side of the auriculo^ 
ventricular plug — that is to say, it expands more at the 
expense of the right than of the left duct of Cuvier, so that 
the latter remains relatively the longer vessel. With the 
final development of the liver and the posterior vena cava 
and the disappearance of the lateral vitelline and subintes- 
tinal veins, the sinus venosus becomes drawn out posteriorly 
and so acquires the irregular pear-shaped formation of the 
adult chamber. The sinu-auricular aperture and the develop- 
ment of the pulmonary vein and fold and their relations to 
the sinus have already been described in detail; it will suffice 
to re-state that the pulmonary vein passes from right to left 
in the roof of the sinus, and arches down obliquely to the left 
in a fold across its anterior wall to reach the left auricle. In 
the adult the sinus venosus is dorsal to the heart, and its 
ventral, anterior and lateral walls have a covering of pericar- 
dium while its dorsal wall is continuous with that structure. 
The closing' of the pericardiac space in front of the liver 
behind the heart causes a contraction externally dividing 
the posterior vena cavui from the sinus venosus. This con- 
traction is represented in the interior of the sinus by a little 
projecting ledge. The subintestinal and lateral vitelline 
veins open into the sinus only for a short period, and the 
ducts of Cuvier, the posterior vena cava and the coronary 
vein are the only vessels permanently connected with it. 
Inferior Jugular, Subclavian and Lateral Cuta- 
neous Veins. — The inferior jugular veins appear at Stage 
31 as little v^essels passing ventrally to the branchial arches,, 
backwards along the lateral surfaces of the pericardium, and 
opening on to the ventral surfaces of the anterior cardinal 
veins at the outer ends of the ducts of Cuvier (Text-fig. 24 e,. 
Inf. J.), These vessels are homologous with those of the 
same name in the Urodeles. The subclavian veins appear 
