Specimen

Specimen
 

-John's writing on essay number five, and his corpus contains a letter addressed to him. He knows that he is behaving like a mad scientist now using human beings around him as specimen but [...]

Is it correct to use those red words and specimen simply but not a specimen/specimens/the specimen?

I think it strange to say 'on essay number five'. The preposition isn't needed.

specimens (must agree in number with beings)

I agree with Rusty on both points.

The only possible meaning that I could accept for "John's (= John is) writing on essay number five" is equivalent to "John is writing about essay number five". In other words, the subject (or topic) of the document that John is writing is a different document called "essay number five".

Excellent answers. But I think the writer thought this: he has chosen number four out of a list of topics. Then do you think 'essay' should be 'question' and 'write on question number four' is correct?

Then I would use 'about', not 'on'.