280 RICHARD E. SCAMMON 



I have not found that the cavities in the mesotheUal tubules or 

 cysts of the selachian hver connect with the blood spaces. As 

 the walls of these structures break up into mesenchymal strands 

 it is impossible with ordinary section and staining methods to 

 distinguish between them and the endothelium surrounding the 

 adjoining sinusoids. This is particularly true if one tries to follow 

 the mesenchymal strands which penetrate between the hepatic 

 cylinders in later stages such as the one shown in figure 38. 

 The mesenchymal and endothelial cells form free anastomoses. 

 McGill ('10) apparently found the same condition in a much 

 younger embryo, judging from figure 9 of her paper with which 

 the section shown in my figure 7 is in agreement. 



The reduction in the thickness of the mesothelial covering of 

 the liver and the disappearance of the mesothelial villi is no doubt 

 due in part to the great growth of the contents of the former, 

 particularly of the hepatic cylinders and sinusoids. This in- 

 crease takes place rather suddenly in embryos from 18 to 20 mm. 

 in length and the reduction in the thickness of the mesothelium 

 is to some extent coincident with it. It is noticeable, however, 

 that in many cases when a hepatic cylinder comes in contact 

 with the covering mesothelium the latter layer is distinctly 

 reduced in thickness, its constituent cells- change from a high 

 columnar to a cuboidal form and the nuclei from oval to nearly 

 spherical bodies. This change is probably not due to pressure 

 for in places where two tubules come in contact with the meso- 

 thelium and are separated by a distance hardly equal to their 

 own diameter the mesothelium becomes thin at the points of 

 contact and remains thickened over the small intervening portion. 

 Were the effect of tubule contact only a matter of pressure, one 

 might expect some flattening of this intervening place as well. 

 Also the mesothelium is not pushed outward at these points of 

 contact but remains on a level with the surrounding surface. 



One is tempted to suggest the homology of this dorsal mesen- 

 chyma-producing mass with the 'Vorleber' of Kolliker and His. 

 But this similarity is only a superficial one as the mass just 

 described does not appear until after the liver is well developed, 



