THE LATER DEVELOPMENT OF THE FROG 



167 



fore-gut, beneath the yolk-mass (Fig. 37). This rudiment 

 enlarges very slowly at first, the solidity of the yolk preventing 

 its penetration. The liver lies just posterior to the heart and 

 separated from it only by a mass of scattered mesoderm cells, 

 which come to be added to the anterior wall of the liver diver- 

 ticulum, forming its mesodermal components. Some of the 

 yolk cells adjoining the liver appear to be added to it, forming 

 true hepatic cells. After hatching, the wall of the anterior 

 part of the diverticulum 

 becomes folded, and later 

 forms the chief part of 

 the definitive liver (Fig. 

 58, A). The postero- 

 ventral extension of the 

 diverticulum is the rudi- 

 ment of the gall-bladder, 

 which becomes somewhat 

 separated from the ante- 

 rior hepatic portion; the 

 original opening of the 

 diverticulum out of the 

 fore-gut remains as the 

 bik-duct (Figs. 58, 59). 



In later stages the liver 



FIG. 59. Models of the digestive tract 

 of frog embryos. After Ham mar (Maurer.) 

 A. Lateral view of the tract of a 7 mm. larva. 

 The anterior portion has been opened by a 

 median sagittal section. B. Dorsal view of 

 the tract of an 8.5 mm. larva, d, Ductus 

 choledochus; g, gall bladder; h, liver; I, 

 enlarges Very considerably lung; m, mid-gut; p, pancreas; pd, dorsal 

 End Shifts its position pOS- rudiment of Pancreas r, rectum. 



teriorly; the gall-bladder also becomes very large in the 

 tadpole. 



The pancreas develops in the region where the . liver diver- 

 ticulum originally opens out of the fore-gut (Fig. 59). It 

 arises from three rudiments. A dorsal rudiment appears as a 

 solid outgrowth of the dorsal wall of the fore-gut, from which 

 it soon separates entirely. Right and left ventral rudiments 

 grow out from the fore-gut, just in the posterior margin of the 

 opening of the bile-duct. These ventral rudiments, retaining 

 a common connection with the gut, then enlarge and, passing 

 around the sides of the bile-duct, fuse together in front of it. 



