Sentence:  Recently , for example , a paranoid woman 's large-scale philosophizing , in the session , about the intrusive curiosity which has become , in her opinion , a deplorable characteristic of mid <WSD>twentieth</WSD> century human culture , developed itself , before the end of the session , into a suspicion that I was surreptitiously peeking at her partially exposed breast , as indeed I was . 
Meaning of the word marked with <WSD> tags:  coming next after the nineteenth in position