In 1841 "fourth class" headright certificates of 640 acres for family heads and 320 acres for single men were granted conditionally to residents who immigrated to Texas between January 1, 1840, and January 1, 1842. A total of 36,876,492 acres was granted by the republic in headright certificates. In order to attract settlers, the Republic of Texas also made colonization contracts with various individuals to establish colonies in the republic and receive payment in land. In addition to large grants made directly to the contractors, settlers in such colonies were granted 640 acres each, if heads of families, or 320 acres, if single. Land grants made under colonization contracts amounted to 4,494,806 acres. As a further inducement to settlers, in 1845 the Congress of the republic passed the first Pre-emption Act, which gave to persons who had previously settled upon and improved vacant public lands, or who might thereafter settle upon and improve them, the right to purchase (pre-empt) up to 320 acres. Pre-emptors, or homesteaders, were required to cover their locations with valid certificates within three years. Under the state government this period was extended to January 1, 1854.