I30 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



vein. The left descending aorta is larger than the right. The ventral aorta may be traced 

 cranially in the series to the fourth aortic arches. The pulmonary artery, if followed caudad, 

 connects with the fifth aortic arches as in Fig. 133. 



Section through the Fifth Aortic Arches (Fig. 133).— The fifth aortic arch is com- 

 plete on the left side. From these pulmonary arches small pulmonary arteries may be traced 

 caudad in the series to the lung anlages. The esophagus, now separate from the trachea, 

 forms a curved horizontal sht. All four chambers of the heart are represented, but the 

 aorta and pulmonary artery are incompletely separated by the right and left bulbar swellings 



or folds. 



Section through the Sinus Venosus and the Heart (Fig. 134). — The section is 

 marked by the symmetrically placed atria and ventricles of the heart and by the presence of 



Spiiuil f,iin I 

 Noloclioi il- 



R. descending noiia 

 Sinus venosus 



R. valve of sinus venosus 



Pericardial cavily 



R. venlrieh 

 Interventricular scpliiu 



\p I il ord 



Upper limb bud 

 Esophagus 



L. common cardinal vein 



Trachea 



L. alritim 



Endocardial cushion 



lUidy wall 



Fig. 134. — Transverse section through the sinus venosus of the heart in a 10 mm. pig embryo. X 22.5. 



the upper limb buds. Dorsal to the atria are the common cardinal veins, the right vein forming 

 part of the sinus venosus. The sinus venostis drains into the right atrium through a slit-like 

 opening in the dorsal and caudal atrial wall. The opening is guarded by the right and left 

 valves of the sinus venosus, which project into the atrium. The septum primum completely 

 divides the right and left atria at this level, which is caudal to the foramen ovale and the 

 atrio-ventricular openings. The septum joins the fused endocardial cushions. Note that 

 the esophagus and trachea are now tubular and that the left descending aorta is much larger 

 than the right. Around the epithelium of both trachea and esophagus are condensations of 

 mesenchyma, from which their outer layers are differentiated. 



Section through the Foramen Ovale of the Heart (Fig. 135). — The level of this 

 section is cranial to that of the previous figure and shows the septum primum interrupted dor- 

 sally to form the foramen ovale. Each atrium communicates with the ventricle of the same 



