In the Black Yajur Veda the Taittiriya Aranyaka is only a continuation of the Taittiriya Brahmana, and the conclusion of the Aranyaka is formed by the Taittiriya Upanishad and the Maha Narayan Upanishad. In the great Satapatha Brahmana of the White Yajur Veda, the first third of Book XIV is an Aranyaka, while the end of the book is formed by the greatest and most important of all Upanishads, the Brhadaranyaka Upanishad. The Chandogya Upanisad , the first section of which is nothing but an Aranyaka, belongs to a Brahmana of the Sama Veda, probably the Tandya Maha Brahmana. The so-called Jaiminiya Upanishad Brahmana is an Aranyaka of the Jaiminiya or Talavakara School of the Sama Veda, and the Kenopanishad, also called Talavakara Upanishad, forms a part of it.
With, the exception of the Maha Narayana Upanishad, which was only added to the Taittiriya Aranyaka at a later period, all the above-named Upanishads belong to the oldest works' of this kind.
Literary Style of Early Upanishads
In language and style they resemble the Brahmanas, component parts of which they are, or to which they are immediately attached. It is the same simple prose, but- especially in the narrative portions- by no means lacking in beauty. Although each one of the great Upanishads contains the earlier and later texts side by side, the age of each individual piece must be determined separately. Yet even the later portions of the above-mentioned Upanishads may claim great antiquity, if only on linguistic grounds.
It may be taken that the greater Upanishads, like the Brhadaranyaka and the Chandogya Upanishad, originated in the fusion of several longer or shorter texts which had originally been regarded as separate Upanishads. This would also explain the fact that the same texts are sometimes to be found in several Upanishads. The individual texts, of which the greater Upanishads are composed, all belong to a period which cannot be very far removed from that of the Brahmanas and the Aranyakas, and is before Lord Buddha and before Panini. For this reason the six above-mentioned Upanishads- Aitareya, Brhadaranyaka, Chandogya, Taittiriya. Kausitaki and Kena- undoubtedly represent the earliest stage of development in the literature of the Upanishads. They contain the so-called Vedanta doctrine in its pure, original form.