O Conqueror of sleep (Arjuna), I am the Self seated in the core of the very ideational process of all beings. I am the beginning, middle and also the end of all beings.
Self is the sole focus of Krishna’s exposition, every time, a point readers and seekers often forget or set aside. Whatever be the name or description used, the reference is to the Self.
Besides panca-bhutas, the internal factors in creation are mind, intelligence and ego. The Self is the substratum of all these. The Supreme can then be only the Self, which reveals everything through the senses, mind and intelligence. No other means of perception is available to the human.
Krishna first points that the Supreme inheres, as the Self, in the ideational process of all beings. Whatever mobile or immobile is expressed before us through birth of any kind, is the Supreme alone. Self is equally the middle and the end of all beings. Locate the Supreme in your own inner recess, understand that all expression and existence is the Self alone.