TRANSVERSE SECTIONS OE A TEN MM. PIG EMBRYO 137 



aorta and the internal jugular vein. At this level the jugular vein receives the linguofacial 

 vem.^ 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. 128. 



Section through the Fifth Aortic Arches (Fig. 128).— The fifth aortic arch is 

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

 caudad in the series to the lung anlages. The cavity of the pharynx forms a curved horizontal 

 slit. 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. 129).— The section is 

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



Spinal card 



% 

 fUescaorta >gL» gfo^L- LOejcenJir,*) 



Trachea 



/?. atrium 



Cayifyof 

 Bulbus 



y 





ft. ventricle y^^^^^^&^^a^^^^^ 



Fig. 128. — Transverse section through the fifth pair of aortic arches and bulbus cordis of 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 venosus 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 

 atria-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. 130). — 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 



