They must have extended over at least two hundred years, and contained long lists of teachers.
Mueller supposes that writing was unknown when the Rig-Veda was composed.
The thousand and ten hymns of the Vedas contain no mention of writing or books, any more than the Homeric poems.

There is no allusion to writing during the whole of the Brahmana period, nor even through the Sutra period.

This seems incredible to us, says Mueller, only because our memory has been systematically debilitated by newspapers and the like during many generations.

It was the business of every Brahman to learn by heart the Vedas during the twelve years of his student life.

The Guru, or teacher, pronounces a group of words, and the pupils repeat after him.
After writing was introduced, the Brahmans were strictly forbidden to read the Vedas, or to write them.