FROM TWELVE TO THIRTY-SIX SOMITES 



179 



floor of the laryngotracheal groove directly continuous with the 

 floor of the branchial portion of the pharynx at its hind end; the 

 former bends up at about right angles to enter the narrow 

 oesophagus (Figs. 87 and 88). 



Thus the whole pulmonary tract communicates widely with 

 the pharynx at the 35 s stage. Its complete delimination falls 

 within the period covered by Chapter X. The continuity of 

 the expansions that form the lung primordia, with the series of 

 visceral pouches as shown in Fig. 100, is especially noteworthy 

 as suggesting a theory of the phylogenetic derivation of the lungs. 



a.i.p. 



Oes. 



FIG. 103. Reconstructions of the liver diverticula of the chick. 

 (After Hammar.) 



A. On the third day of incubation; from the left side; the divertic- 

 ula arise from the anterior intestinal portal. 



B. Beginning of the fourth day; from the left side. 



a. i. p., Anterior intestinal portal. D. V., Indicates position of 

 ductus venosus. g. b., Gall bladder. 1. d. d. (cr.)., Dorsal or cra- 

 nial liver diverticulum. 1. d. v. (caud.), Ventral or caudal liver 

 diverticulum. pc. d., Dorsal pancreas. X., Marks the depression in 

 the floor of the duodenum from which the common bile duct is 

 formed. 



(Esophagus and Stomach. Immediately behind the pharynx, 

 at the stage of 36 s, the intestine narrows suddenly (primordium 

 of oesophagus) and enters a small, spindle-shaped enlargement, 

 the primordium of the stomach (Figs. 87, 88; 100). 



The liver arises in the chick as two diverticula of the entoderm 

 of the anterior intestinal portal, one situated immediately above 

 and the other below the posterior end of the ductus venosus, or 

 fork of the omphalomesenteric veins (Fig. 103 A). This portion 



