408 THE BABBIT. 



the thyroid body also moves its position, coming into close 

 relation with the upper rings of the trachea. 



3. The Thymus. 



The thy m us of the rabbit is formed by bud-like outgrowths 

 from the epithelium of one of the hinder branchial pouches. 

 These buds first become conspicuous about the fourteenth day ; 

 they soon separate from the walls of the pharynx, and gradually 

 shift backwards, increasing greatly in size as they do so, until 

 they reach their final position at the anterior end of the thorax. 



4. The Lungs. 



The lungs arise in the rabbit, much as in the chick or frog, 

 from the ventral wall of the mesenteron, at the place where 

 it narrows, immediately behind the pharyngeal region, to form 

 the oesophagus. 



On the tenth day the cavity of the oesophagus, which is 

 elsewhere circular in transverse section, becomes laterally com- 

 pressed at its anterior end, immediately behind the pharyngeal 

 region. By the outgrowth of two horizontal ridges from its 

 side walls, which meet and unite in the median plane, a short 

 length of the oesophagus becomes divided into two tubes : of 

 these, the dorsal tube is the oesophagus itself; while the ventral 

 one, or laryngeal chamber, is a short tube, ending blindly behind , 

 but opening in front into the oesophagus through the orifice 

 which afterwards becomes the glottis (c/. Fig. 150). 



From the laryngeal chamber the lungs arise, on the eleventh 

 day, as a pair of lateral diverticula, which grow backwards along 

 the dorsal part of the body cavity and the sides of the oesophagus 

 (Fig. 150, LG). 



The lungs, being thus formed as outgrowths from the ali- 

 mentary canal, will, like the canal itself, have mesoblastic walls, 

 lined by a hypoblastic epithelium. 



On the^ twelfth day secondary outgrowths arise from the main 

 tube or bronchus of each lung, and these in the later stages 

 branch freely to form the smaller bronchi, from the terminal 

 branches of which the air cells are formed about the time of birth. 



The branchings of the bronchi occur almost entirely towards 

 the dorsal and outer surfaces of the lungs (Fig. 163), so that the 

 original or main bronchial tubes, LB, lie close to the inner surfaces 



