The artificial incubation of hens' eggs involves four factors—temperature, humidity, air supply, and at intervals a rotation or ‘turning’ of the eggs. This last factor is perhaps the most curious and unexpected of the four, yet there is no doubt that it is necessary for development as shown by good hatchability, and in the natural state it is carried out by the sitting hen. Eycleshymer (1906), Chattock (1925), and Olsen (1930) have all concluded from observations on the hen's nest that the hen frequently rotates the eggs during the incubation period; Olsen considers it occurs as often as 96 times in 24 hours.

Various abnormalities have been recorded in eggs incubated without turning. Dareste (1891) stated that absence of turning causes the allantois to adhere to the yolk sac; Eycleshymer (1906) confirmed this, and added that during the first week of incubation, absence of turning may also cause the embryo to adhere to the shell membranes.

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