PLATE 4 



CXPLANATION OF FIGURES 



41 Showing a rounded cell (m) projecting into the pericardial cavity from 

 the visceral pericardium, but still attached bj- a slender cytoplasmic pedicle to 

 the mesothelial surface. 7 mm. pig embrj^o (cf. fig. 17, 42, 44). 



42 Demonstrating the potentiality for phagocytic activity on the part of 

 the mesothelial cell. The mesothelial character of the cell can hardly be ques- 

 tioned, at the same time it is partially raised above the level of the mesothelial 

 surface and the nucleus approximates a kidney-shaped form. From the parietal 

 pericardium of a 9 mm. pig embryo. 



43 Showing the position and general relation.s of the section of the pleuro- 

 pericardial membrane (pp) drawn at a larger magnification in figure 14. It may 

 he observed that the cellular mass in question lies in the pleural cavity {pcv) 

 and that the latter is still in communication with the pericardial cavity through 

 the pleuro-pericardial canal {pplc). The membrane continues through fourteen 

 sections and is found to connect with the parietal wall at the junction of parietal- 

 pleural (ppl) and parietal-pericardial (ppc) walls where its cells become contin- 

 uous with the mesothelium lining the coelomic walls. 



44 Showing a region of the visceral pericardium in which the mesothelial 

 cells present the appearance of transformation into free cellular elements. This 

 region continues through several successive sections. Many of the cells assume 

 a more basophilic stain than that of typical mesothelial cells, and phagocytic 

 activities, cytoplasmic vacuolation and peripheral processes or buds (6) appar- 

 ently identical with that of the coelomic macrophags. 7 mm. pig embryo. 



122 



