What determines the duration of pregnancy

A.S. Hagedoorn and A. L. Hagedoorn (according to Ber. U. D. G. Gyn. Bd. XIV, H. 8) cite observations of rabbits and hares. It is known that pregnancy in rabbits lasts 4 weeks. Shortly before giving birth, the rabbit prepares a nest and displays her cubs in it; young rabbits are naked and blind. In hares, the duration of pregnancy is 8 weeks; hares are born larger than rabbits, sighted, covered with hair and can move freely. When a rabbit is mated with a hare, naked, blind rabbits similar to each other are born, in an adult state, the instinct of a rabbit is found in females (they prepare nests), and pregnancy lasts 4 weeks. When a hare is mated with a rabbit, pregnancy lasts 8 weeks, animals are born, similar to rabbits, but covered with wool, sighted, much larger and capable of movement. From these observations, the authors conclude that the duration of pregnancy and the term of delivery do not depend on the development of the fetus and not on its hormones, this, apparently, is the reaction of the maternal organism, which is determined by the genotype. This reaction, apparently, depends on the development of a special tissue or group of cells in the mother's body, since sometimes a non-pregnant rabbit, after mating, prepares a nest and covers it with wool.