276 RICHARD E. SCAMMON 



a length of 15 mm. the mesothelium is smooth and much reduced 

 in thickness. When the embryo reaches a length of 18 mm. the 

 villi are reduced to small conical projections and are absent in 

 embryos of 20 mm. Thereafter the mesothelium covering the 

 ventral part of the liver on both sides becomes steadily reduced 

 in thickness, and in embryos of 25 mm. and longer it is an ex- 

 ceedingly thin layer of squamous epithelium. 



The second and dorsal mesenchymal proliferation begins at a 

 later stage than does the ventral one. In an embryo 6.4 mm. in 

 length (S.C. 19) (fig. 6 A) the dorsal portion of the mesothelium 

 is seen to be slightly thinner than that of the ventral zone. As 

 the lateral pouches of the liver are pushed upward, the omphalo- 

 mesenteric veins come to lie mainly dorsal to these structures 

 and the mesothelium over them is gradually thickened. This 

 thickening is most noticeable at the lateral margins of the dorsal 

 surface (fig 6 B). Soon after, in embryos of 10 to 12 mm., the 

 mesothelium of this region becomes distinctly thickened in places 

 and is invaginated forming tubules the walls of which are continu- 

 ous with the splanchnic mesothelium and the lumina with the coe- 

 lomic cavity (fig. 6 C and fig. 7). Often these tubules first appear 

 as long trenches on the coelomic surface of the mesothelium. 

 The sides of these trenches coalesce, thus roofing over the de- 

 pression and forming tubules which open into the coelom at both 

 ends. Other tubules grow in and end blindly. Examples of 

 these early tubules are shown in figure 8. Although fairly 

 regular at the start, these structures soon become irregular in 

 caliber and form, and very often anastomose. Their lumina 

 may be occluded in places thus forming mesothelial cysts which 

 are generally connected by solid stalks with the covering meso- 

 thelium. At the time of their highest development the tubules 

 form an anastomotic network which fills the dorsal fifth of the 

 anterior part of the liver and they are then easily mistaken at 

 first sight for true hepatic tubules (fig. 6D). More careful 

 study shows a number of differences between the two structures. 

 Aside from the nuclear differences to be mentioned later, the 

 mesothelial tubules are more irregular than the hepatic cylinders, 

 their walls are thinner both actually and relatively as compared 



