TUMORS. 



311 



mesoderm. Embryonic endothelial cells are structurally iden- 

 tical with embryonic connective tissue cells. These tumors are 

 not very common in domestic animals. An endothelioma was 

 observed in the lung of a dog, another in the testicle of a bull. 

 These tumors may have their origin from the endothelium lining 

 blood or lymph channels, peritoneum, pleura, pericardium, arach- 

 noid membrane, any organs developed from mesothelium, or 

 deflections from any of them. They are varial)le in shape, size, 

 color and consistency. 



Microscopically they are composed of cells that most fre- 





■Sl^ 



* <a 0. e*^ 



4I 



,2er<» 





«* 





. 7j'oi//y'/^£~ 



Fig-. 166.- 



-Section of Endothelioma from a bull's testicle. Note the hands of 

 connective tissui; and arrangrement of cells. 



quently resemble sarcoma cells, although they may approximate 

 the structure of carcinoma cells. The cells may be arranged 

 in tubules, transverse or oblique sections appearing as sectii.ms 

 of gland tubules or acini. If arranged in columns transverse or 

 oblique sections appear as cell nests. The cells are usually 

 cubical or spherical in shape, although they may be spindle or 

 even squamous. The stroma \'aries according to the tissue in- 

 vaded and may be dense fibrous or mucoid. Blond vessels are 

 usually quite numerous, and if the endothelium is derived from 



