THE ANATOMY OF A SIX MM. PIG EMBRYO 



97 



with it to form the closing plates. Between the ventral diverticula of the 

 third pair of pouches lies the median thyreoid anlage. The fourth pouch 

 is smaller than the others. Its dorsal diverticulum just meets the ecto- 

 derm; its ventral portion is small, tubular in form, and is directed parallel 

 to the esophagus (Fig. 104). 



The groove on the floor of the pharynx caudal to the epiglottis is 

 continuous with the tracheal groove. More caudally, opposite the atrium 



Eye 



Maxillary process 



Mouth 



Branchial arch 3 

 Branchial arch 4 



Upper limb bud 



Hepatic diverticulum 



Yolk sac 



Body stalk. 



Allantois 



Umbilical artery 



Mesonephric duct 



Pronto-nasal process 







'Olfactory pit 



Mandibular process 

 Branchial arch 2 

 Aortic bulb 



Trachea 

 Lung bud 



Stomach 



Cephalic loop of intestine 

 Mesonephros 

 Mesonephric duct 

 Caudal loop of intestine 



'Lower limb bud 



-Rectum 



Dorsal aorta and umbilical artery 

 FIG. 97. Ventral dissection of a 6 mm. pig embryo. The head has been bent dorsally. X 14. 



of the heart, the trachea has separated from the esophagus (Fig. 96). The 

 trachea at once bifurcates to form the primary bronchi and the anlages'of 

 the lungs (Fig. 97). The lungs consist merely of the dilated ends of the 

 bronchi surrounded by a layer of splanchnic mesoderm. They bud out 

 laterally on each side of the esophagus near the cardiac end of the stomach, 

 and project into the pleural ccelom. The esophagus is short and widens 

 dorso-ventrally to form the stomach. The long axis of the stomach is 

 nearly straight, but its entodermal walls are flattened together and it has 

 revolved on its long axis so that its dorsal border lies to the left, its ventral 

 border to the right, as seen in transverse section (Fig. in). 



7 



