SB 1.3.12

tataḥ saptama ākūtyāṁ
 rucer yajño ’bhyajāyata
sa yāmādyaiḥ sura-gaṇair
 apāt svāyambhuvāntaram
Synonyms: 
tataḥ — after that; saptame — the seventh in the line; ākūtyām — in the womb of Ākūti; ruceḥ — by Prajāpati Ruci; yajñaḥ — the Lord’s incarnation as Yajña; abhyajāyata — advented; saḥ — He; yāma-ādyaiḥ — with Yāma and others; sura-gaṇaiḥ — with demigods; apāt — ruled; svāyambhuva-antaram — the change of the period of Svāyambhuva Manu.
Translation: 
The seventh incarnation was Yajña, the son of Prajāpati Ruci and his wife Ākūti. He controlled the period during the change of the Svāyambhuva Manu and was assisted by demigods such as His son Yāma.
Purport: 

The administrative posts occupied by the demigods for maintaining the regulations of the material world are offered to the highly elevated pious living beings. When there is a scarcity of such pious living beings, the Lord incarnates Himself as Brahmā, Prajāpati, Indra, etc., and takes up the charge. During the period of Svāyambhuva Manu (the present period is of Vaivasvata Manu) there was no suitable living being who could occupy the post of Indra, the King of the Indraloka (heaven) planet. The Lord Himself at that time became Indra. Assisted by His own sons like Yāma and other demigods, Lord Yajña ruled the administration of the universal affairs.