86 V. E. EMMEL 



pleuro-pericardial and pleuro-peritoneal membranes. These 

 membranes as can be directly observed, are localized centers of 

 increased mitosis and cellular growth. Without taking into ac- 

 count here the deeper lying causes it appears evident that this 

 increased cellular proliferation is an important factor in the 

 gradual outward extension of the coelomic walls or membranes, 

 the final fusion of the juxtaposed surfaces of which is destined 

 to effect a closure of the pleuro-pericardial and pleuro-peritoneal 

 canals. In such regions of fusion the free mesothelial surfaces 

 necessarily disappear. Consequently, if mesothehal cells can 

 differentiate into coelomic macrophags such regions might be 

 expected to furnish valuable evidence of such a process. 



Figure 43 is one of fourteen sections of such a region in a 7 mm. 

 pig embryo, showing the embryonic pleural cavity (pc) and the 

 pleuro-pericardial canal (pplc). At the center of the figure is 

 seen a section through the distal border of the pleuro-pericar- 

 dial membrane (pp) for the left pleuro-pericardial canal (the 

 embryo having been cut in the sagittal plane). Figure 14 shows 

 the same central mass drawn at a higher magnification. If this 

 cellular mass is traced back through the fourteen sections in 

 which this membrane is present to its attachment to the parietal 

 wall, its component cells are found to merge and become con- 

 tinuous with the mesothelial and mesenchymal elements at the 

 juncture of the parieto-pleural and parieto-pericardial walls. 

 Directing, attention more especially to the present object of in- 

 quiry it is important to note that toward the more peripheral or 

 distal margins of these advancing membranes the superficial cells 

 have a more nearly spherical shape and that a definite flattened 

 mesothelium is no longer evident. In the more central portion 

 of the section shown in figure 14 there may be recognized a some- 

 what more definite layer of mesothelial cells {??ies) surrounding a 

 central core (c). Toward the periphery of the section, however, 

 the cells are no longer so intimately united and many of them 

 are partly if not completely detached as free rounded cells. 

 The structural characteristics of such an area is not indicative 

 of degenerative changes. On the contrary the frequent mitotic 

 figures (m) furnish ample proof of active cell multiplication and 



