Sentence:  Your invitation to write about Serge Prokofieff to honor his 70 th Anniversary for the April issue of Sovietskaya Muzyka is accepted with pleasure , because I admire the music of Prokofieff ; and with sober purpose , because the <WSD>development</WSD> of Prokofieff personifies , in many ways , the course of music in the Union of Soviet Socialist Republics . 
Meaning of the word marked with <WSD> tags:  a process in which something passes by degrees to a different stage (especially a more advanced or mature stage)