332 OUTLINES OF CHORDATE DEVELOPMENT 



A). This groove is the beginning of the whole pulmonary 

 system. The depression becomes well marked early the third 

 day as the laryhgo-tracheal groove, and its posterior end 

 expands transversely forming the rudiments of the lungs. The 

 groove then becomes cut off from the pharyngeal cavity as 'the 

 rudiment of the trachea, remaining open out of the pharynx 

 only at its anterior end; this opening is the glottis, behind which 

 the larynx develops later. From the eighth to the eleventh 

 days the glottis and larynx are obstructed by a cell mass, and 

 the glottis itself remains closed for some time longer. 



Just in front of the glottis, in fact both immediately behind 

 and in front of the thyroid rudiment, the floor of the pharynx is 

 elevated (fourth day), the two papillae thus formed representing 

 the beginning of the tongue. As these rudiments enlarge they 

 fuse together, the thyroid having been cut off meanwhile, and 

 grow forward into the buccal cavity, finally extending nearly to 

 the tip of the jaws. 



The bifurcated posterior extremity of the laryngo-tracheal 

 groove or lung rudiment, grows backward through the sur- 

 roupding mesenchyme; the tubes thus formed are the rudiments 

 of the bronchi. By a sort of budding process they form the 

 bronchioles and terminal alveoli of the lung proper. The meso- 

 dermal parts of the entire pulmonary tract are derived from the 

 splanchnic mesoderm of the primary gut-wall. The air-sacs 

 are also formed from terminal dilations of branches of the 

 bronchial tubes. 



Passing backward from the pharynx the gut is considerably 

 narrowed for a short distance as the cesophagus, but it soon 

 expands again as the rudiment of the stomach (third day). 

 Before these organs are differentiated, however, the liver 

 appears. This makes its appearance toward the close of the 

 second day, in the region where the fore-gut is at that time 

 open by the anterior intestinal portal, i.e., just back of the 

 future posterior limit of the stomach. Here two evaginations 

 of the ventral wall of the gut, or portal, push out, one above 

 the other in the mid-line ; they extend forward as pouch-like out- 

 growths, through the mesoderm of the ventral mesentery, below 



