ii8 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



palatine tonsil. The third pouch is tubular, directed at right angles to the phar- 

 ynx, and meets the ectoderm to form a "closing plate." Median to the plate, the 

 ventral diverticulum of the third pouch is the anlage of the thymus gland. Its 

 dorsal diverticulum forms an epithehal body, or parathyreoid. The fourth 

 pouch is smaller and its dorsal diverticulum gives rise to a second parathyreoid 



Gang. nn. 7 and 8 Gang. n. 5 

 Otocyst 



Pharyngeal pouch 2 

 Gang, jiigularc n. 10 



Aorlic arch j 



Pharyngeal pouch ; \ 



Caudal root 11. 

 hypoglossus 

 Gang. Froricp 



Aorlic arch 4 

 Gang. ccrv. i 



Pharyngeal pouch 4 



Aortic arch 5 



R. descending aorta 



Esophagus 



Trachea 



Vertebral artery 

 Subclavian artery 

 R. lung 



Dorsal pancreas 



Vitelline arterv 



Ventral pancreas 



Descending aorta 



Post, lobe hypophysis 



Ant. lobe hypophysis 



Pharyngeal pouch i 

 iIa.\iUary process 

 Thyreoid gland 

 Pulmonary artery 

 lorta 

 1 ( tk sac 

 R ventricle 



Septum 

 transversum 



Liver 



Hepatic 

 diverticulum 

 Cloaca 

 AUanlois 

 Rectum 



Metancphros 

 Umbilical artery 



llcsoncphric duct 

 Notochord Cephalic limb infest, loop 



Fig 123.-Reconstruction of a 10 mm. pig to show the position of the various organs from the right 

 ■ side. The vems are not indicated. Broken hnes indicate the outline of the left mesonephros and the 

 positions of the limb buds. X 10. 



body. Its ventral diverticulum is a rudimentary thymus anlage. A tubular 

 outgrowth, caudal to the fourth pouch, is regarded as a fifth pharyngeal pouch 

 in human embryos and forms the ultimobranchial body on each side (see p. 164) 

 The thyreoid gland, composed of branched cellular cords, is located in the mid- 

 line between the second and third branchial arches (Fig. 123). 



