154 THE FROG. 



dition lasts for a little time after the opening of tlie moiitli ; and 

 then, in tadpoles of about 10| mm. length, the lumen is gradu- 

 ally re-established, though it is for a time exceedingly narrow. 



This blocking up of the oesophagus, which prevents any food 

 getting into the digestive part of the alimentary canal until some 

 little time after the mouth opening is established, is a curious 

 developmental feature ; it occurs also in the chick and in many 

 other Vertebrates, but its meaning has not yet been explained 

 satisfactorily. 



7. The lungs. 



The lungs arise as a pair of pouch-like diverticula of the 

 side walls of the CESophagus, shortly before the hatching of the 

 tadpole ; they are at first exceedingly small, and have strongly 

 pigmented walls. After hatching, the lungs increase slowly in 

 size, growing backwards along the sides of the oesophagus ; in 

 9 mm. tadpoles, at the time when the oesophagus is solid, the 

 lungs are present as a pair of lateral outgrowths immediately 

 behind the oesophageal plug (Fig. 64, to), but sometimes arising 

 from the solid part itself. After the re-opening of the oesophagus, 

 the part of the ventral wall from which the lung sacs arise be- 

 comes depressed to form the laryngeal chamber : the mouth of 

 the' depressed portion narrows to form the glottis, and the lungs 

 themselves rapidly increase in size. 



In 12 mm. tadpoles, in which the hind limbs are just ap- 

 pearing (Figs. 65 and 75), the glottis is a narrow slit-like opening, 

 guarded in front by a well-developed epiglottis, and leading into 

 a large laryngeal chamber (Fig. 65, Lc), from which the two lungs 

 arise ; these latter are thin-walled vascular sacs (Fig. 76, LG), 

 which now reach to the hinder end of the body cavity, lying 

 along the sides of the alimentary canal. 



From their mode of development as outgrowths of the oeso- 

 phagus, it follows that the lungs are lined by an epithelium 

 which is of hypoblastic origin ; the connective tissue and vascular 

 elements of the lung wall are, like those of other parts of the 

 body, mesoblastic. 



8. The Liver. 



About the time of first appearance of the nervous system, the 

 yolk-mass becomes marked oiF in front by a deep, backwardly 

 projecting depression (Fig. 60, l), from the thin-walled anterior 



