Sentence:  MPs pushed back the boundaries of science last night when they voted to allow the creation of <WSD>hybrid embryos</WSD>, which have a combination of human and animal DNA, as well as "saviour siblings" to save sick children. 
Meaning of the word marked with <WSD> tags:  Specifically, an embryo created artificially which combines human and animal DNA, in various proportions, for the purposes of medical research.