Endothelioma. 381 



of the dura mater of the brain and spinal cord. EndotheHomata 

 in their structure stand relatively midway between the atypical 

 connective tissue growths and epithelial tumors, and resemble now 

 one and now the other of these so closel}- as to make their distinc- 

 tion from sarcomata or carcinomata often practically impossible 

 where only single bits of die growths are subjected to micro- 

 scopic examination. They dilTer from cancers in the history of 

 their development, the latter taking their origin from epithelium, 



v if '• • ' '' ' " c f- .f 5^ 



_ _ ". , _ ^. Jt, «», -K. ^t 









Fig. 111. 



Endothelioma of lymphgland (human): a, lymphoid tissue of gland; h, leucocytes 

 in nearly obliterated lymph-sinus ; c, endotlielial cells proliferated from lining 

 of lymph-sinus. 



either ectodermal or entodermal, while the true endotheliomata 

 are of mesoblastic origin. It is more difficult to define them 

 from sarcomata, the cells of the latter being also derived from the 

 middle germinal layer ; and for this reason some authors speak 

 of them as endothelial sarcomata. (The layer of so-called endo- 

 thelial cells lining the large serous cavities of the body, the coelo- 

 matous lining, is to be regarded- as epithelial and the tumors 

 which arise from these cells are therefore to be classed as 

 cancers.) 



