Have a look here. http://www.helium.com/items/638456-tips-for-increasing-reading-comprehension-for-college-students...
In particular, where it says
Topic sentences, however, are not always placed at the start of each paragraph. When they are, they create what are known as loose or open paragraphs. Topic sentences can also be found in the middle (which creates what is known as a balanced paragraph) and they can be found as the last sentence of a paragraph (which creates an end-stopped paragraph). So another technique which students found useful was to ask themselves the question: Is the paragraph I am going to read an OPEN, BALANCED OR END-STOPPED one?