Srimad Bhagavatam Canto 09, Chapter 06, Text 02

SB 9.6.2

rathitarasyaprajasya
 bharyayam tantave ’rthitah
angira janayam asa
 brahma-varcasvinah sutan
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Rathitara had no sons, and therefore he requested the great sage Angira to beget sons for him. Because of this request, Angira begot sons in the womb of Rathitara’s wife. All these sons were born with brahminical prowess.
 
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
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 ksetra-jata. Because Rathitara had no sons, he took advantage of this method.
Srimad Bhagavatam Canto 09, Chapter 06, Text 01
Srimad Bhagavatam Canto 09, Chapter 06, Text 03