RECONSTRUCTION MODEL OF A HORSE EMBRYO TWENTY-ONE DAYS OLD. 341 



The Vascular System. 



The Heart. — The heart includes (l) a large sinus venosus ; (2) a sinu-atrial canal ; 

 (3) a ventricle; (4) an atrioventricular canal; (5) a bulbus cordis; and (6) a 

 truncus aorticus. 



The sinus venosus is large. It forms the most caudal part of the heart, and lies 

 in the curve of the bent embryo. 



It is somewhat cuboidal in form. It is in relation dorsally, and near the median 

 plane, with the ventral wall of the fore-gut, and, on each side, with the ventral wall 

 of the corresponding pleuro-pericardial canal, into which it bulges (text-figs. 25, 26, 

 below and p. 342, and fig. 58, PI. XIX). 



Spinal Medulla 



Dorsal Aorta 



Wall of Pleuro-pericardial 

 Canal 

 ■ Post. Cardinal Vein 

 Pleuro-pericardial Canal 

 Sinus Venosus 



■Amnion 



Sinus Venosus 



Post&nlinal Vein 



Pericanlial Parity^ 

 Vitelline Vein 



Umbilical Vein-'' 

 Wolffian Body' 



Amnion' \ 



Spi/ialMedulla : 



Dorsal Aorta. 



-"Umbilical Vein 

 VitellineVein 



- Post Cardinal Vein 



Umbilical Vein 

 "Wolffian Body 



WusclePIate 



Text-fig. 25. — Oblique section through the embryo along the line 1 , fig. 58, PI. XIX. 



On account of the curvature of the embryo the ventral part of the sinus venosus 

 has a comparatively reduced cranio-caudal extent, as contrasted with the dorsal part ; 

 and at its caudal end, near the median plane there is a slight evagination which 

 may possibly be the rudiment of the cranial end of the inferior vena cava, but at 

 this stage the evagination in question has no connection with any venous channel in 

 the liver region. The cephalic end of the sinus opens through a short and wide 

 sinu-atrial canal into the caudal part of the atrium. The position of this canal is 

 marked on the exterior, on each side, by a very definite sulcus (fig. 57, PI. XIX). 



The tributaries which open into the sinus are the two vitelline veins, the two 

 umbilical veins, and the two ducts of Cuvier. Each vitelline vein enters at the 

 caudal end of the corresponding lateral border of the sinus. The terminations of the 

 umbilical veins are close to those of the vitelline veins, but are situated a little more 

 medially and more dorsally. The short duct of Cuvier, on each side, enters the cranial 

 end of the sinus on its dorsal aspect and near the lateral border (fig. 58, PI. XIX). 



