THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



forming the tympanic membrane. The second pouch later largely dis- 

 appears; about it, develops the palatine tonsil. The third pouch is tubular, 

 directed at right angles to the pharynx, 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 epithelial body, or parathyreoid. The fourth pouch is smaller and its 



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



Otocyst 



Pharyngeal pouch 2 

 Gang, jugular e n. 10 



Aortic arch 



Pharyngeal pouch 3 



Caudal root n. 

 hypoglossus 

 Gang. Frorie i 



Aortic arch 4 

 Gang. cerv. i 



Pharyngeal pouch 4 



Aortic arch 6 



R. descending aorta 



Esophagus 



Trachea 



Vertebral artery 

 Subclavian artery 

 R. lung 



R. atrium 



Stomach 



Post, lobe of hypophysis 



Ant. lobe of hypophysis 



Eye 



Pharyngeal pouch i 

 Maxillary process 

 Thyreoid gland 

 Pulmonary artery 



Aorta 



Yolk sar 



R. ventricle 



Septum 

 transversum 



Liver 



Hepatic 

 diverticulum 

 Cloaca 

 Allantois 

 Rectum 



Ureter 



Dorsal pancreas 



Vitelline artery 



Ventral pancreas 



Descending aorta, 



Mesonephric duct 

 Notochord Cephalic limb intest. loop 



Metanephros 

 Umbilical artery 



FIG. 123. Reconstruction of a 10 mm. pig, to show the position of the various organs from 

 the right side. The veins are not indicated. Broken lines indicate the outline of the left 

 mesonephros and the positions of the limb buds. X 10. 



dorsal diverticulum gives rise to a second parathyreoid 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. 

 165). The thyreoid gland, composed of branched cellular cords, is located 

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



