Srimad-Bhagavatam: Canto 9 - Chapter 1 - Verse 2
रथीतरस्याप्रजस्य भार्यायां तन्तवेऽर्थित: ।अङ्गिरा जनयामास ब्रह्मवर्चस्विन: सुतान् ॥ २ ॥
rathītarasyāprajasyabhāryāyāṁ tantave ’rthitaḥaṅgirā janayām āsabrahma-varcasvinaḥ sutān
Rathītara had no sons, and therefore he requested the great sage Aṅgirā to beget sons for him. Because of this request, Aṅgirā begot sons in the womb of Rathītara’s wife. All these sons were born with brahminical prowess.
In the Vedic age a man was sometimes called upon to beget sons in the womb of a lesser man’s wife for the sake of better progeny. In such an instance, the woman is compared to an agricultural field. A person possessing an agricultural field may employ another person to produce food grains from it, but because the grains are produced from the land, they are considered the property of the owner of the land. Similarly, a woman was sometimes allowed to be impregnated by someone other than her husband, but the sons born of her would then become her husband’s sons. Such sons were called kṣetra-jāta. Because Rathītara had no sons, he took advantage of this method.