What is the textual structure of original Pali versions of the suttas of the Majjhima Nikaya?
The Dhamma was traditionally spoken, heard, memorized and recited in unison. It was written down and organized later.
Bhante Sujato has a very useful overview of the challenge of organizing the Dhamma throughout the ages. Notably, Bhante Sujato has himself chosen to organize the Tipitaka into numbered segments, which allows translations to be matched up with their root sources verse by verse.