'''iri the Foetus of Vertehrated Animals. 261 



passages are developed on the anterior side of the oesophagus, 

 but they render it improbable that these organs are produced, 

 as some have supposed, like a process or diverticulum of the 

 intestinal canal. According to Rathke, the lungs are formed^^ 

 short time before the trachea or bronchi ; they consist, on the 

 fourth and fifth days, of two small mucus-like masses (Fig. 26, 

 A. B.) situated above the pericardium, and before the stomach. 

 The trachea and bronchi {!*) appear at first like a thickening 

 of the lower side of the oesophagus ; the trachea is at first so 

 short, proportionally to the bronchi, as to be scarcely perceptible. 

 On the fifth day these parts are increased in size (Fig. 27), and 

 have become more distinct, but they are as yet quite solid, and 

 without any internal cavity. On the sixth day, \he trachea is 

 elongated proportionally to the bronchi, and a cavity is evident 

 in the interior of the lungs. This cavity, however, is confined 

 to the posterior and lower part of the lungs only, and Rathk^ 

 has shown that this part (Fig. 28, I) afterwards becomes the 

 cellular part of the respiratory organs of the birds. The ante-' 

 rior part (L), corresponding to the bronchial part of the lun^ 

 still remains quite solid. On the seventh day, the cavity in the . 

 cellular part of the lung is increased, and the trachea and* 

 bronchi are become quite hollow; minute air-tubes radiating 

 from the extremities of the bronchi, at the same time, appear in* 

 the anterior solid part. After this period the whole lung grows, 

 rapidly, and approaching the vertebral column and ribs, becomes 

 firmly united to these parts. The bronchial part of the lung re^'^ 

 mains closely united with the air-sacs till the 12th day (Fig. 29)^^^ 

 when the vesicular part begins to increase with great rapidity, 

 and envelopes all the viscera of the chest and abdomen a few 

 days before the chick comes out of the egg. ''' u^ i'^f^/ 



The communication of these air-sacs with the boiles is^not^ 

 cstabhshed till some days after the end of incubation. 



Each of the lungs, shortly after its formation, receives an 

 arterial branch from the pulmonary arches (the fourth lefV M^^ 

 the fifth right branchial arches), (Fig. 30, p). These branches' ' 

 gradually become larger as the lungs are developed ; but as the 

 blood which they carry to the lungs is returned to the left auricl^ ,^ 

 by proper veins, the parts of the arches leading into the aorta 

 behind the pulmonary arteries gradually become less. These 



