268 RICHARD E. SCAMMON 



section of an embryo about 4 mm. longer of the same species 

 shows the complete establishment of the hepatic network. 



While the early tubules of Raia, Mustelus and Torpedo are 

 smaller in cross-section than are those of Acanthias, the lumina 

 of these tubules are considerably larger. The cells lining the 

 lumina are cubical or low columnar in outline as compared with 

 the high columnar type found in Acanthias. The nuclear struc- 

 ture is quite similar to that of the hepatic cells in Acanthias, the 

 chromatin covered nucleoli described for that form being par- 

 ticularly prominent in Torpedo and Mustelus. 



The later history of the cylinders is quite similar to that of 

 Acanthias. The cylinders rapidly increase in diameter and the 

 contained lumina become smaller. The increase in diameter 

 of the cylinders is due, as in Acanthias, to the growth of the 

 individual cells, and the number of cells about the lumen in 

 any given section steadily decreases. The nuclear changes are 

 similar to those found in Acanthias. In such specimens of jVIus- 

 telus and Squatina as I have examined the fat contained in hepatic 

 cells remains in discrete droplets instead of forming one large 

 mass as is generally the case in Acanthias. The same is true to 

 some extent of Torpedo (fig. 36). 



Pilliet ('90) has described areas of comparatively undifferenti- 

 ated cells in the adult selachian liver. These cells form the 

 portions of the cylinders which lie about the hepatic-portal veins. 

 They are of comparatively small size and contain centrally 

 placed nuclei which stain deeply with alum-carmin. The fat 

 content of the cells is less than of those cells located elsewhere, 

 and particularly of those lying in the neighborhood of the hepatic 

 veins. Pilliet regards these smaller cells as reserves or nests of 

 young cells which, from their position near the nourishing vessels, 

 contribute to the growth of the organ. Apparently he did not 

 find them in all the specimens which he examined. 



I have seen no evidence of a retardment of the differentiation 

 of the cells near the hepatic-portal veins in Acanthias embryos, 

 and if such occurs it must be at a late period in the development 

 of the organ. There are numerous small cells in the immediate 

 neighborhood of the hepatic-portal vessels, but they all form 



