I 3 



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 

 epithelium. The first, second, and third branchial arches show on either side of the section, 

 the third being slightly sunken in the cervical sinus. The dorsal diverticula of the third 

 pharyngeal pouches extend toward the ectoderm of the third branchial groove. 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 



'Spinal cord 



Int. jugular vein 

 N. hypoglossus 

 Gang, nodosum n. 10 



Pharyngeal pouch j 



Aortic arch 3 

 Thyreoid anlage 



Spinal ganglion 



Notochord 

 Ext. branch n. accessorius 



Epiglottis 

 Branchial arch j. 



Branchial arch 2 

 Mandible 



Olfactory pit 



FIG. 131. Transverse section through the third pharyngeal pouches of a 10 mm. pig embryo. 



X 22.5. 



Olfactory epithelium 



order, are the internal jugular -veins, the hypoglossal nerve, and the nodose ganglion of the 

 vagus. Lateral to the 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 



