Sentence:  It is now recognized that <WSD>crossfeeding</WSD> does not occur with derivatives of the mouse L-cell line (such as A9) , nor with certain HeLa clones. However, <WSD>crossfeeding</WSD> does occur with other heteroploid mouse lines (3T3 and derivatives), hamster BHK-21 cells and derivatives, and diploid human fibroblasts. 
Meaning of the word marked with <WSD> tags:  To coexist on a single limiting resource where one strain or species grows on the primary resource and the second grows on the partially degraded resource excreted by the first.