CELLULAR ELEMENTS OF THE MAMMALIAN EMBRYO 85 



The important question next arises as to the fate of these 

 mesotheUal cell masses. How long they may persist as defi- 

 nitely organized structures the present data does not indicate. 

 While mitotic figures were not observed neither was there any 

 evidence of degeneration noted. That their component cells 

 may assume phagocytic activities is indicated in the smaller 

 of the two masses in figure 15. But the point especially perti- 

 nent to our present purpose is the fact that some of these masses 

 show evidence of further modification and subsequent disinte- 

 gration into isolated free cells. In the smaller of the two groups 

 shown in figure 15 the nuclei are more rounded in shape and the 

 epithelial character of the component cells is no longer so clearly 

 evident. The single isolated cell in the same figure is appar- 

 ently of the same type except that the nucleus has assumed a 

 kidney shape and the cytoplasm a deeper basophilic stain. These 

 changes seem strikingly shown in figure 16 in which several of 

 the peripheral cells have become more spherical in form and the 

 cytoplasm takes a darker stain. Of the two approximately 

 spherical cells in the lower part of the figure, one is partially and 

 the other almost entirely free from the main mass. Such cells 

 appear quite comparable to the basophilic cells or macrophags 

 found free in the coelomic cavities. 



Granting the correctness of the above conclusions, these dis- 

 integrating mesothelial cell masses, therefore, furnish a type of 

 evidence indicative of the cytological transformation of meso- 

 theUal cells into coelomic macrophags which appears to obviate 

 the possible objections previously noted with reference to simi- 

 lar changes at the surface of the coelomic wall. For it is evident 

 that the rounded cells at the periphery of these masses can cer- 

 tainly not be regarded as migratory cells from mesenchymal or 

 vascular sources and it is highly improbable that they represent 

 free coelomic macrophags incidentally resting or fixed at the 

 surface of the mass. 



c. The mesothelium of the pleuro-pericardial and pleuro-peri- 

 toneal membranes. The second of the two sources of evidence 

 referred to on page 84 is found in the regions concerned with the 

 subdivision of the embryonic coelom by the development of the 



