Tm; !)I(;I-:STIVE TRACT 589 



which is carried forward by the anterior growth of the tongue, so that 

 the glandular portion becomes the tip of the tongue. 



The Liver is one of the earliest diverticula of the alimentary tract, 

 appearing even before the embryo itself has begun to elongate. It lies 

 beneath the yolk mass and develops from the ventral portion of the fore- 

 gut, just posterior to the heart. A group of scattered mesodermal cells 

 lies between' liver and heart, which will soon be added to the anterior 

 wall of the liver rudiment. The anterior portion of the liver becomes 

 folded after hatching. 



The Gall-bladder is formed as a postero-ventral extension of the 

 liver diverticulum which becomes more or less separated from the an- 

 terior portion of the liver. 



The Bile-duct (Fig. 332) is the original opening of the liver divertic- 

 ulum into the alimentary tract from which it grew. 



The Pancreas (Figs. 343, 293) develops in close proximity to the 

 liver diverticulum from three separate rudiments, a dorsal and two ven- 

 tral. The dorsal rudiment is a solid outgrowth from the dorsal wall 

 of the fore-gut. A complete separation between outgrowth and origin 

 soon takes place. 



The right and left ventral rudiments grow out of the fore-gut at the 

 posterior margin of the bile-duct. These retain their connection with 

 the gut, enlarge, and after passing around the bile-duct, fuse together 

 in front of it. The dorsal portion later also fuses with this fused right 

 and left portion and connects with the gut by the pancreatic duct. At 

 this period, the pancreatic duct forms the boundary between the fore- 

 gut and the mid-gut, although later the pancreatic duct comes to lie 

 within the margin of the bile-duct. Oesophagus and stomach develop 

 quite as they do in the chick. It will be remembered that the oesophagus 

 closes for a time in the chick. It does likewise in the frog just after 

 hatching, when the tadpole is about eight millimeters in length. 



The oesophagus, however, again opens just before the mouth is 

 formed, that is, when the tadpole is about ten or eleven millimeters 

 long. 



The stomach is at first longitudinal, but it soon bends, and comes 

 to lie transversely, a matter of importance when nerves are to be traced, 

 for the nerves and blood vessels which are paired and known as right 

 and left will now be dorsal and ventral, depending upon the side toward 

 which the caudal end of the stomach turns. 



THE DERIVATIVES OF THE MID-GUT 



As with the chick, the mid-gut is confined to that more or less cen- 

 tral space where the yolk continues being absorbed and converted into 

 other substances in the growing embryo and tadpole. After hatching, 

 the yolk is absorbed very rapidly, some of the yolk-cells becoming the 

 epithelial lining of the growing intestine. The intestine is bent into a 



