HISTOGENESIS OF THE LIVER 273 



cylinders. The origin of the hepatic mesenchyma can be followed 

 with little difficulty in Acanthias. Here it appears that this 

 material is probably derived completely from the mesothelium 

 but at two distinct periods and from two different regions, and 

 that in both cases this proliferation is associated with distinct 

 irregularities of splanchnic mesothelium. 



The first mesenchymal proliferation appears at a little later 

 time than the formation of the stroma of that portion of the gut 

 which lies posterior to the liver. In Acanthias embryos 5 mm. 

 in length the liver pouch is but slightly differentiated and con- 

 sists of two shallow diverticula which lie in the lateral walls of 

 the archenteron and are fused anteriorly to form the median 

 hepatic pouch. The archenteron of this region is clothed on 

 either side by a layer of splanchnic mesoderm which is continuous 

 above with the radix mesenterica and below with the splanchnic 

 layer of the blastoderm. At the radix this layer is much thickened 

 but it is reduced to a moderately thin layer over the sides of the 

 archenteron. The irregular endothelial walls of the omphalo-- 

 mesenteric veins intervene between the ventral part of the 

 archenteron and the mesothelium, but no mesenchyma is present. 

 Irregular processes from the mesothelium, however, do extend 

 inward and in places it appears as though cells were about to 

 be delaminated. A short time later the hepatic pouch increases 

 much in size and extends anteriorly far in front of the anterior 

 intestinal portal. With this change the ventral parts of the 

 investing layers of splanchnic mesothelium are brought in con- 

 tact and eventually fuse, thus forming for some time a ventral 

 mesentery. In connection with this process there appear two 

 distinct sets of mesothelial irregularities. These consist of the 

 mesothelial villi on the right side and of numerous irregular folds 

 on the left. 



The mesothelial villi found in connection with the covering 

 of the selachian liver were first described by Choronshitzky ('00), 

 although they were observed in other forms long before that 

 time. Hochstetter ('00) has given their later history in Acanthias 

 in connection with his study of the formation of the septum 

 trans versum. As Hochstetter has stated, these structures are 



