Sentence:  But after the doctor 's return that night Alex could see , from the high window in his own room , the now familiar figure crouched on a truly impressive heap of towels , apparently giving its egg hatching powers one final chance before it was <WSD>replaced</WSD> in its office by a sure-enough hen . 
Meaning of the word marked with <WSD> tags:  substitute a person or thing for (another that is broken or inefficient or lost or no longer working or yielding what is expected)