128 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



Section through the Third Pharyngeal Pouches (Fig. 131).— The tip of the head 

 is now small and shows on either side the deep olfactory pits lined with thickened olfactory epi- 

 thelium. The first, second, and third branchial arches show oh either side of the section, the 

 third being shghtly sunken in the cervical sinus. The dorsal diverticula of the third pharyn- 

 geal pouches extend toward the ectoderm of the third branchial cleft. The ventral diverticula 

 or thymic anlages may be traced caudad in the series. The floor of the pharynx is sectioned 

 through the epiglottis. Ventral to the pharynx are sections of the third aortic arches and the 

 solid cords of the thyreoid gland. Dorsally the section passes through the spinal cord and 

 first pair of cervical ganglia. Between the cord and pharynx, named in order, are the internal 

 jugular veins, the hypoglossal nerve, and the nodose ganglion of the vagus. Lateral to the 



Spinal giiHg/; '/' 



Notocbord 

 Ext. branch n. accessoi ins 



Epiglollis 

 Branchial arcli j. 



Branchial arch 



Mandible 



Olfactory pit. 



Spinal cord 



Int. jugular vein 

 A', hypoglossus 

 Gang, nodos. n. lo 



Pharyngeal pouch 3 



Aortic arch 3 

 Thyreoid antage 



Olfactory epithelium 



Fig. 131. — Transverse section through the third pharyngeal pouches of a 10 mm. pig embryo. X 22.5. 



ganglion is the external branch of the n. accessorius, and mesial to the ganglia are. the small 

 descending aorta. 



Section through the Fourth Pharyngeal Pouches (Fig. 132). — This region is marked 

 by the disappearance of the head and the appearance of the heart in the pericardial cavity. 

 The tips of the atria are sectioned as they project on either side of the bulbus cordis. The 

 bulbus is divided into the aorta and pulmonary artery, the latter connected with the right 

 ventricle, which has spongy muscular walls. The pharynx is crescentic and continued laterally 

 as the small fourth pharyngeal pouches. Into the mid-ventral wall of the pharynx 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 linguo-facial 



