V THE DEVELOPMENT OF THE FROG 107 



arise as outgrowths from it. The first of these to be formed 

 is the liver, which at the beginning appears as an outpocket- 

 ing of the ventral side near the anterior end. The out- 

 pocketing becomes folded and branched, being converted 

 finally into a number of clusters of tubules, all emptying 

 into the common canal, the bile duct, which is produced 

 by a lengthening of the neck of the original outgrowth. A 

 lateral outgrowth of the bile duct forms the gall bladder. 

 The cells lining the terminal branches of the hepatic diver- 

 ticula become the secreting cells of the liver. The connec- 

 tive tissue, blood vessels, and outer coating of the liver are 

 derived from the mesoblast. 



T\\& pancreas d.x\^^% much in the same way as the liver, 

 but as a pair of outgrowths instead of a single one. They 

 form, however, a single organ, and their ducts later become 

 connected with the bile duct. Only the secreting portion 

 of the pancreas and the lining of its ducts are of entodermic 

 origin, the connective tissue, blood vessels, etc., arising, as 

 in the liver, from the middle germ layer. 



The bladder arises as an outgrowth of the ventral side 

 of the alimentary canal, near the posterior end ; its lining, 

 therefore, is of entodermic origin. 



The lungs appear as a pair of pouches from the sides of 

 the esophagus. They make little growth until quite late in 

 the Hfe of the tadpole. The region of the esophagus from 

 which the lungs arise becomes depressed and partly sepa- 

 rated off from the part above to form the larynx, the mouth 

 of the depressed portion going to form the glottis of the 

 adult. 



The gill slits in the frog appear in the form of five solid 

 outgrowths on each side of the anterior portion of the 

 archenteron. In section they are shown to be in the form 

 of a double fold such as would be produced if the walls of 



