268 



ZOOLOGY. 



sented in both milk- and permanent sets, a second enamel- 

 organ is budded off from the first to form the permanent 

 tooth. 



2. Respiratory Organs. The mesenteron is not as 

 sharply divided into respiratory (pharyngeal) and digestive 

 (intestinal) regions in chick and rabbit as in the tadpole 

 or Amphioxus ; but the two regions can be recognized. 

 From the former arise the gill-slits, with arches between 

 them, much as in the tadpole, though no gills are ever 

 developed from them. So also do the thyroid, the thymus, 

 and the lungs. The origin of the two former has been 

 mentioned under Amphioxus (chap, xvi., 15), and need not 

 be further described. The lungs arise in the chick during 

 the third day of incubation (and at a corresponding stage in 

 the rabbit) as a median ventral diverticulum of the mesen- 

 teron, just behind the gill-slits. Its distal portion almost 

 immediately becomes paired, giving rise by growth and 

 repeated branching to the bronchi, bronchioles and alveoli ; 



while the proximal median 

 part elongates into trachea 

 and larynx. Thus the whole 

 of these structures are lined 

 by hypoblast. The cartilages, 

 etc., in the walls of the trachea 

 and larynx are, of course, 

 formed from splanchnic meso- 

 blast (fig. 142). 



3. The Liver and Pancreas 

 are formed at about the same 

 period as the lungs, by out- 

 growths of the digestive por- 

 tion of the mesenteron (fig. 

 142). The liver arises as a 



(From Balfour, after Gotte) Hypoblast ^y Q f ven t ra l outgrowths 

 black, splanchnic mesoblast dotted. * ; ,. wwg*vww*w 



which later (in the rabbit) 



become united into one (in the frog there is only one at the 

 first). These grow out and branch in a most complicated 

 manner, the branching of what will be the actual liver 



Fig. 142. PART OF MESENTERON OF 

 EMBRYO CHICK. 



