CELLULAR ELEMENTS OF THE MAMMALIAN EMBRYO 87 



there is abundant evidence of phagocytic activities (pcm), intra 

 cellular digestion and vacuole formation (vcm). It may be fur- 

 ther observed in these regions that the more superficial cells are 

 not only modified in form, but that the cytoplasm also tends to 

 take a deeper basophilic stain. Such cells as icm and pcm are 

 certainly identical in structure with the coelomic macrophags. 

 At the same time there are present all transitional stages be- 

 tween these cells and the original mesothelial elements. Fi- 

 nally beginning at these proliferating areas, detached free cells 

 of the same type may be traced through successive sections out 

 into the adjacent body spaces as they become scattered through- 

 out the pericasdial, pleural and peritoneal cavities where they 

 appear indistinguishable from the free cells normally present in 

 these regions. Evidently here again, just as in the case of the 

 free mesothelial cell masses previously described, the proliferat- 

 ing and disintegrating mesothelium is giving rise to free cells 

 functioning as macrophags in the coelomic cavities. 



In resume it may be stated, therefore, that the data derived 

 from both the pleuro-pericardial and pleuro-peritoneal mem- 

 branes and the free mesothelial cell masses support in a sub- 

 stantial manner the conclusion suggested by the cytological con- 

 ditions observed at the surface of the .coelomic walls. Namely, 

 that the coelomic mesothelium is an important source of the 

 phagocytic cells found in the embryonic coelom, and that as 

 these mesothelial cells round up and become detached from 

 various regions of the coelomic wall, they assume structural and 

 functional characteristics identical with that of the coelomic 

 macrophags typical of these embryonic body cavities. 



IV. ERYTHROCYTIC ELEMENTS IN THE COELOMIC CAVITIES 



As already indicated (p. 76) the second group of cellular coe- 

 lomic elements are characterized by their eosin staining quali- 

 ties and non-phagocytic activity. These eosin staining ele- 

 ments again fall into two sub-groups, the one consisting of small 

 non-nucleated bodies and the other of larger nucleated cells. 



