THE ORGANS OF THE INNER GERM-LAYER. 329 



duodenum, which receives the two ductus hepatici. The evagination 

 gradually increases to a long single canal, the bile-duct or ductus 

 choledochus, the result of which process is that the whole liver is 

 farther removed from its source of origin. 



By an evagination either of the ductus choledochus or of one of 

 the two ductus hepatici, the gall-bladder with its ductus cysticus is 

 established. In Man it arises from the ductus choledochus, and is 

 present as early as the second month. 



The network of hepatic cylinders, which are sometimes hollow, 

 sometimes solid, is metamorphosed in two ways. 



One part becomes the excretory ducts (the ductus biliferi). In 

 the cases in which the hepatic cylinders are at first solid, they begin 

 to become hollow and to arrange their cells into a cubical or cylin- 

 drical epithelium around the lumen. In this process some of the 

 branches of the network must degenerate. For, whereas all hepatic 

 cylinders at first communicate with one another by means of anas- 

 tomoses, this is, as KOLLIKER remarks, no longer the case in the 

 adult, except at the outlet of the liver (Leberpforte), where the 

 well-known network of bile-ducts exists. 



The remaining part of the network furnishes the secretory paren- 

 chyma of liver-cells. The character of a netlike tubular gland, 

 which becomes so evident during development, is to be recognised 

 even in the fully developed organ in the case of the lower Verte- 

 brates, the Amphibia and Reptiles. The tubules of the gland, 

 which were from the beginning hollow, subsequently exhibit an 

 exceedingly narrow lumen, which is demonstrable only by means of 

 artificial injection, and which in cross section is surrounded by three 

 to five liver-cells. Through their manifold anastomoses they produce 

 an extraordinarily fine network, the small meshes of which are filled 

 up by a network of capillary blood-vessels, together with a very small 

 amount of connective substance. 



In the higher Vertebrates (Birds, Mammals, Man) the tubular 

 structure of the gland subsequently becomes very inconspicuous and 

 the liver acquires a complicated structure, information concerning 

 the details of which is given in the text-books of histology. 



There are three things which, from a developmental point of view, are not to 

 be lost sight of : first, the capillaries of the bile-duct have arisen by canalisa- 

 tion of the primitive hepatic cylinders ; secondly, they are bounded by only 

 cwo liver-cells, which are very large and flake-like ; thirdly, they send out 

 evaginations between and even into the liver-cells themselves. In this way ? 

 greater complication is brought about in the arrangement of the fine biliary 



