As I discuss in this answer, the Universe lasts for one Kalpa or day of Brahma. Each Kalpa is divided into fourteen Manvantaras, periods where a given Manu governs the race of humans. Now we are presently governed by Surya's son Vaivasvata Manu, but during the first Manvantara of the present Kalpa, humanity was governed by Brahma's son Swayambhuva Manu. The Srimad Bhagavatam describes Swayambhuva Manu's children:
[I]n due course of time he [Manu] begot in Śatarūpā five children — two sons, Priyavrata and Uttānapāda, and three daughters, Ākūti, Devahūti and Prasūti. The father, Manu, handed over his first daughter, Ākūti, to the sage Ruci, the middle daughter, Devahūti, to the sage Kardama, and the youngest, Prasūti, to Dakṣa. From them, all the world filled with population.
Devahuti's husband Kardama was a sage born from Brahma's shadow. And Prasuti's husband Daksha was born from Brahma's thumb. But my question is, who is this sage Ruchi who married Akuti? He's presumably a son of Brahma, but the only named sages who had been created by Brahma are the four Sanat Kunaras, Kardama, and the Prajapatis. Yet only ten Prajapatis who are listed earlier on in the chapter:
Marīci, Atri, Aṅgirā, Pulastya, Pulaha, Kratu, Bhṛgu, Vasiṣṭha, Dakṣa, and the tenth son, Nārada, were thus born.
In any case, Ruchi's status as a Prajapati is confirmed later on in the Srimad Bhagavatam, in a description of Ruchi's son, the incarnation of Vishnu known as Yagna:
Ākūti had two brothers, but in spite of her brothers, King Svāyambhuva Manu handed her over to Prajāpati Ruci on the condition that the son born of her be returned to Manu as his son. This he did in consultation with his wife, Śatarūpā. Ruci, who was very powerful in his brahminical qualifications and was appointed one of the progenitors of the living entities, begot one son and one daughter by his wife, Ākūti. Of the two children born of Ākūti, the male child was directly an incarnation of the Supreme Personality of Godhead, and His name was Yajña, which is another name of Lord Viṣṇu.
Vishnu incarnated as Ruchi's son Yagna in order to fill the position of Indra during the Manvanthara of Swayambhuva Manu. So are there any scriptures which describe the birth of the father of this important incarnation?