178
Śrīmad-Bhāgavatam
[Canto 9, Ch. 6]
PURPORT
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.
TEXT 3
एते क्षेत्रप्रसूता वै पुनस्त्वाङ्गिरसाः स्मृताः । रथीतराणां प्रवराः क्षेत्रोपेता द्विजातयः ॥ ३ ॥
ete kṣetra-prasūtā vai punas tv āṅgirasāḥ smṛtāḥ rathītarāṇām pravarāḥ kṣetropetā dvi-jātayaḥ
ete —the sons begotten by Aṅgirā;  kṣetra-prasūtāḥ —became the children of Rathītara and belonged to his family (because they were born from the womb of his wife);  vai —indeed;  punaḥ —again;  tu —but;  āṅgirasāḥ —of the dynasty of Aṅgirā;  smṛtāḥ —they were called;  rathītarāṇām —of all the sons of Rathītara;  pravarāḥ —the chief;  kṣetra-upetāḥ —because of being born of the  kṣetra  (field);  dvi-jātayaḥ —called  brāhmaṇa  (being a mixture of  brāhmaṇa  and  kṣatriya ).
TRANSLATION
Having been born from the womb of Rathītara's wife, all these sons were known as the dynasty of Rathītara, but because they were born from the semen of Aṅgirā, they were also known as the dynasty of Aṅgirā. Among all the progeny of Rathītara, these sons were the most prominent because, owing to their birth, they were considered  brāhmaṇa .