Sentence:  In the present paper an attempt is made to find for the tetrahedron the analogues of the <WSD>circles of Apollonius</WSD> of the triangle. 
Meaning of the word marked with <WSD> tags:  (geometry) The figure (a generalised circle) definable as the locus of points "P" such that, for given points "A", "B" and "C", <small>\frac{|AP|}{|BP|} = \frac{|AC|}{|BC|}</small>.