274 RICHARD E. SCAMMON 



found only on the right side. They appear in embryos of 

 45 to 30 segments as a thickened plate of the mesothelium 

 overlying the right omphalo-mesenteric vein (fig. 6 A) . This 

 plate is soon thrown into a series of pouch like irregularities, and 

 the spaces thus formed are filled with a delicate network of 

 protoplasmic processes from the mesothelial cells. The cores 

 of the villi are thus at no time really empty, and are soon occupied 

 by delaminated mesenchymal cells. From the bases of these 

 villi mesenchymal cells are proliferated apparently both from the 

 walls and the mesenchymal core, and soon come to form a small 

 mass on the right side lying just below the omphalo-mesenteric 

 vein. On the opposite side the irregularities are not villi but 

 longitudinal folds. Unlike villi they do not arise from a thickened 

 plate, the mesothelium at this point remaining as thin as else- 

 where at the time of formation. Afterward the cells become 

 more columnar and a mass of mesenchyma, larger but similar 

 in other respects to that of the opposite side, is proliferated and 

 underlies the left omphalo-mesenteric vein. Figure 7 shows 

 the position and extent of these two mesenchymal proliferations 

 in an embryo 7.5 mm. long (H.E.C. 1496). Soon after this 

 stage the ventral mesentery breaks down and the ventral surface 

 of the liver is free throughout its extent. With this process 

 there is a delamination of mesenchymal cells along the median 

 ventral line which unites the lateral ones already described, and 

 there is thus formed a general ventral bed of mesenchyma which, 

 as growth proceeds, forms a coating about the gall bladder and 

 constitutes a large loose-meshed mass which extends forward 

 from the gall bladder to the anterior mesothelial wall of the 

 liver. 



This constitutes the first and ventral contribution of mesen- 

 chyma to the liver from its mesothelial envelope. In its entire 

 extent it produced the mesenchyma which surrounds the gall 

 bladder, and later the vessels adjacent to it, the covering of the 

 cystic duct, and to an undeterminable degree the sparse mesen- 

 chymal tissue of the lower part of the hepatic parenchyma. 

 The irregularities of the mesothelium on the right side do not 

 continue for any great period. By the time the embryo reaches 



