132 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



opens the vertical slit of the trachea. A section of the vagus complex is located between 

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

 the lingua-facial 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 sixth aortic arches as in Fig. 133. 



Section through the Sixth Aortic Arches (Fig. 133). The sixth aortic arch (see p. 100) 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 esophagus, now separate from the 

 trachea, 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. 



Spinal ganglion 

 Notochord- 



Spinal cord 



R. descending aorta 

 Sinus venosus 



R. valve of sinus mnosus 

 Pericardial cavity 



R. ventricle, 

 Interventricular septum 



Upper limb buo, 

 Esophagus 



L. common cardinal veia 

 Trachea 



L. atrium 



Endocardial cushion 

 Body wall 



FIG. 134. Transverse section through the sinus venosus of the heart in a 10 mm. pig embryo. 



X 22.5. 



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 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 com- 

 pletely 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. 



