31. 



\eti;riiNar\ p.\-]H()Lt.;(.;\-. 



the endothelium of a vessel, the vessel may be very irregular i 

 calibre and structure. If the cells occur in columns or nests i 

 will be necessary to ditTerentiate them from carcinomata. Thi 

 differentiation involves the comparison of cells derived froi 

 mesoderm and those derived from entoderm or ectoderm. Th 

 only essential difference, and that is not constant, is the size c 

 the nucleus. The differentiation may also be governed to som 

 extent by the distribution of the blood vessels. If the cells occu 

 in tubules, their differentiation from the adenoma will be neces 

 sarv. Adenomata mav be differentiated by observing the same 



viL;. li;7, — Fr(jni clra\vini< of a Nodule of a ^Mediastinal endotiielionia. 

 1. Column of .■ndothelial cells. 11. DitEusu mass of endotht-lial cells. 



factors that are used in differentiating endotheliomata from car- 

 cinomata. Alveolar sarcomata are verv difffcult to differentiate 

 from endotheliomata, in fact it is sometimes impossible. 



These tumors are not encapsulated and usuallv form meta- 

 stases. Thev usually occur in internal organs and hence surgical 

 relief is imp(.)ssible. Fatal termination is the usual outcome. 



Fibrosarcoma. — This is a tumor composed of both adult and 

 embryonic connective tissue. They are quite common, espe- 

 cially in the eyelids and in labial commissures of horses and 



