342 



PROFESSORS ARTHUR ROBINSON AND A. GIBSON. 



The Atrium. — Even when the shrinkage (figs. 57, 62, PL XIX) which the atrium 

 has undergone has been fully taken into account, it is obviously a smaller chamber 

 than the sinus venosus. It lies more towards the left than the right side of the 

 embryo, cranial to the sinus venosus and dorsal to the left portion of the ventricular 

 chamber (figs. 57, 58, 62, PL XIX). Its dorsal wall lies close to the ventral wall of the 

 fore-gut, in the region of the ventral ends of the second and third branchial pouches, 

 and, as it bulges towards the fore-gut, it is largely responsible for the elevation which 

 forms the ridge-like caudal boundary of the groove which connects the ventral ends of 

 the second branchial pouches together (figs. 57, 63, PL XIX). The ventral wall of the 

 atrium lies in relation with the dorsal wall of the left portion of the ventricle, into 

 which it opens through a short but distinct atrio-ventricular canal (fig. 62, PL XIX). 



Spinal Medulla 



Ant. G'trdinal Vein 

 Sinus Venosus. 



FoJdofVite/Im 

 Vein 



Jtudiment or"-, 



Vitel/ineVein^ 

 ZirerBudimen? 



Jiight Dorsal Aorta 



DuctofCurier 

 \.P/euro-perieardial Canal 

 -Sinus Venosus 



•Amnion 



- Vitelline Vein 



)" -Sp7anc7inopJeure 



'"—•l/mbiJical \%in 

 ^Amnion Cavity 



K Musc/ePIate 

 "Dorsal Aorta 

 \Spinal Medulla 



U/nbdicaJ Vein- 

 Wolffian Body / 



Text-fig. 26.— Oblique section through the embryo along the line 2, fig. 58, PI. XIX. 



To the left of the atrio-ventricular canal the left margin of the atrium projects 

 ventrally, forming the rudiment of the left auricle. The right auricular portion of 

 the atrium which lies medial and caudal to the bulbus cordis is not so distinctly 

 deliminated. 



The Ventricle. — The ventricle forms the most ventral portion of the heart. It 

 crosses the median plane from left to right, but lies more to the right than to the 

 left side (figs. 55, 56, 62, PL XIX). Its ventral wall, on the left side, is deeply 

 invaginated, a condition obviously due to shrinkage, for in the drawings of the 

 fresh specimen the ventricular area is uniformly convex. There is already definite 

 surface indication of the separation of the ventricle into right and left portions 

 (fig. 62, PL XIX). 



The bulbus cordis commences at the cranial end of the right ■ segment of the 

 ventricle and it runs medially and dorsally till it reaches the median plane, where it 



