HISTOGENESIS OF THE LIVEK 271 



section and the lumen distinctly enlarges. The cells on the 

 vascular side of the lumen are reduced in actual height but be- 

 come more colimmar in form because of the still greater reduction 

 of the size of their bases. At the same time the nuclei which 

 in hepatic cylinders are round or broadly oval in section and lie 

 near the center of the cells become elongately oval in outline and 

 tend to retreat to the bases of the cells. The cells are so di- 

 minished in size that the nuclei which increase little, if any, in 

 bulk almost fill them. The nuclei lose their typical structure 

 of a clear karyoplasm containing one or two large chromatin 

 masses from which radiate chromatin threads and present in- 

 stead a reticular chromatin network made up of evenly distrib- 

 uted granules of about the same size. The protoplasm becomes 

 homogeneous and colors darkly with plasma stains. An example 

 of such a developing duct at an early stage is shown in figure 39. 

 Approximately one-third of the duct which abuts upon a blood 

 vessel shows considerable progress in differentiation, while the 

 cells of the segment opposite it are true hepatic cells. Bet\/een 

 the two are zones of transitional cells. To the side of this duct is 

 a smaller one in still an earlier stage of differentiation. The out- 

 line sketch shown in figure 5 illustrates the changes in shape of 

 the hepatic cells at the time of duct formation. 



The cells of the large hepatic ducts which are formed from the 

 hepatic pouches and their evaginations show much the same 

 steps in cytomorphosis as do those just described. The nuclei 

 of the larger ducts, however, are oval at the start and so undergo 

 no changes in form, but the change in chromatin arrangement 

 is the same as in the minor ducts. In the gall bladder the same 

 changes also take place but at a late stage (60-80 mm.) the 

 nuclei again become circular in cross section and come to occupy 

 the centers of the cells which are much elongated. 



It is sometimes stated that the bile duct epithelium is formed 

 of cells of a more primitive type than those of the cylinders or 

 trabeculae. In the forms under discussion, however, the bile 

 duct cells have departed farther from the embryonic type than 

 the parencyhmal cells, if we may judge by their nuclear structure. 



