230 GEORGE W. CORNER 



l)ules like those of the pancreas to have openings at both ends 

 when the flow is entirely in one direction. 



Laguesse first found these non-anastomosing buds appearing 

 upon the primitive plexus in sheep embryos, and called them 

 ]:)rimitive pancreatic tubules. At first they terminate in little 

 l)ranching vesicles. Renaut ('99) maintains that there is always 

 one island of Langerhans in each of these clusters of terminal 

 vesicles, and that the whole structure represents a primitive lob- 

 ule, to become in time a fully-developed lobule of the pancreas, 

 the island being, as it were, its center of growth; to which he adds 

 that the islands become rudimentary in the adult, since their 

 foetal function is lost. These ideas of Renaut regarding the 

 islands have not stood the test; but it is true that the lobules 

 arise by growth of the early tree-like outgrowths. 



To return to our series: Up to this point the pancreatic tissue 

 has been relatively homogeneous externally, that is, not lobulated. 

 But as the tree-like growth supervenes upon the duct, the tissue 

 becomes rounded off and divided into irregular little nodules, 

 each one of which represents a branch of the duct with its twigs. 

 Further progress is best studied by watching some definite part 

 of the pancreas, for instance, the 'upper bridge' connecting the 

 head and tail of the organ (fig. 1, UB). In this spot, in the 80 

 mm. foetus (fig. 14) the duct is found giving off branches at 

 varying intervals to little rounded masses of tissue of irregular 

 size, which are fused together at many points. In the 110 mm. 

 foetus (fig. 15) there is a little more semblance of order; the 

 masses of tissue are larger, more discrete, and show, in short, a 

 transition to the formation of units as described in the first part 

 of this paper, which is fairly complete in the 'bridge' at 140 mm. 

 (fig. 16). 



The increase in the number of units during development (actu- 

 ally counted by teasing, in the foetal stages, estimated in the 

 adult) is roughly as follows: 



stage Number of units 



1 15 mm. length 250 



140 mm. lenstli 400 



160 mm. length 700 



■Vdult 20,000 to 80,000 



