Yaagyavalkya and Taittireeya Sanhitaa (Krishn)
Yaagyavalkya Jee was the disciple of Vaishampaayan Jee (and Vaishampaayan Jee was the disciple of Ved Vyaas Jee). When the great sage Ved Vyaas Jee divided Ved in four parts and taught them to his various disciples, he taught Yajur Ved (later called Taittireeya Sanhitaa) to Rishi Vaishampaayan Jee and Rishi Vaishampaayan Jee, in turn, taught that Ved to his disciple Yaagyavalkya Jee.

Once some Rishi jointly decided to have regular meetings and whoever would not attend those meetings will be attached to Brahm Hatyaa sin. Only one Rishi did not attend it and he was Vaishampaayan Jee. In madness of this Vaishampaayan Jee killed his nephew (sister's son) and asked his disciples to do something which could free him from this sin.

At this Yaagyavalkya Jee said to him - "There is no need to trouble all of them for this, they are not so powerful, I alone will do this Vrat." At this Vaishampaayan Jee got angry with Yaagyavalkya Jee that he had insulted other Braahman and asked him to leave everything whatever he had learnt from him. Yaagyavalkya immediately vomited out all the Yajur Ved's Mantra and departed from there. Seeing the Ved Mantra lying on the ground, many other Rishi assumed the form of partridges (Teetar) and ate them up. When they taught these Ved Mantra to their disciples, this branch was called Taittireeya branch of Yajur Ved as it was descended from Teetar or partridges.

Another version says that, when Yaagyavalkya Jee vomited out the Mantra of Yajur Ved, by chance Ved Vyaas Jee had visited his disciple, so he changed some Rishi into partridge and they swallowed those burning coals. Later Ved Vyaas Jee again changed them to Rishi.

Yaagyavalkya and Vaajsaneyee Sanhitaa (Shukla)
Yaagyavalkya was now in search of better knowledge, so he did Tap for Soorya Dev and Soorya Dev pleased from his Tap appeared before him in the form of a horse (Vaaji) and asked Yaagyavalkya to ask for any Var. Yaagyavalkya Jee said - "Prabhu, If you are pleased with me, teach me those Ved Mantra which are not known to anybody." So Soorya Dev imparted him Yajur Ved Mantra from his horse's mouth which were unknown to anyone else, even to Yaagyavalkya's Guru Vaishampaayan Jee. Since these Mantra were delivered through horse's mouth, this was called Vaajsaneyee Sanhitaa (Vaaj means horse). Besides, because it was evolved in great rapidity from Soorya in the form of a horse through his manes that Yaagyavalkya divided this Vaajsaneya Yajur Ved again into 15 branches, each branch comprising hundreds of Yaju's Mantra which came to be known as Vaajsaneya (descended from a horse's mane) branch of the Yajur Ved. [see What is Taittireeya for its detailed explanation] Kanv Rishi, Madhyaandini and others learnt those branches.

Vishwaamitra Jee is believed to be the author of most of the Mandal 3 of Rig Ved, including Gaayatree Mantra. According to Puraan, only 24 Rishi understood the meaning of the Gaayatree Mantra fully and thus wielded the whole power of it - sage Vishwaamitra was the first and sage Yaagyavalkya was the last.

Yaagyavalkya was the one who told Raam Kathaa, in Tulasee Daas Jee written Raam Charit Maanas, to Bharadwaaj Muni and others after they had taken bath in Gangaa for the whole Maagh month. He has written Brihadaaranyak Upanishad also. In Brihadaaranyak Upanishad Yaagyavalkya and Gaargee dialog is very famous.

Another Version of the Above-mentioned Story About Yaagyavalkya
This story about Yaagyavalkya I read on Internet, so I am not sure about its authenticity, however it is another version of the above story. It goes like this --

Yaagyavalkya Rishi was the disciple of Maharshi Jaimini. Once students were sitting, and Maharshi Jaimini was teaching them that a messenger came from the King's court. In those days it was customary for the King to start his day's work after having blessings of Braahman, so Maharshi Jaimini sent Yaagyavalkya for this purpose. The young Yaagyavalkya went to the King's court and sat down. The King was still taking bath, so he had to wait. Yaagyavalkya said to the messenger curtly - "The King should have called me when he was ready. Now I am missing my lessons." The messenger told this to the King. The King told the messenger to ask for "Akshataai" and asked the Braahman to leave. So Yaagyavalkya left the "Akshataai" in the nearby Peepal tree trunk and left the palace for his Gurukul. The tree was cut at the trump level.

When the King came out after taking bath, he was very surprised to see the fully grown up Peepal tree in the place of a stump. The King asked the messenger as what happened. The messenger told him everything. The King understood the power of Rishi and ran towards the Gurukul. He told everything to Maharshi Jaimini; but Guru wanted to admonish the student for his impatience. So he called Yaagyavalkya and asked him that why he could not wait for the King? And as a punishment he asked him to spit whatever he had learnt from him. The student obeyed the Guru. He assumed the form of an ostrich (called Taittireeya) and spat and emitted three chunks of black color. This was called Krishn Yajur Ved. The famous Tattireeya Sanhitaa of the Krishn Yajur Ved was called because of this.

After that he did penance for Soorya Dev and received Shukla Yajur Ved from him.
[see What is Taittireeya for its detailed explanation]