Sources : Hen

Pliny the Elder [1st century CE] (Natural History, Book 10, 74-75): [Book 10, 74] The eggs grow to full size in the uterus in ten days from pairing, but in the case of the domestic fowl and the pigeon, if the hen is disturbed by having a feather torn out or by some similar damage, it takes longer. In all eggs the middle of the yolk contains a small drop of a sort of blood, which people think is the heart of birds, supposing that the heart is the first part that is produced in every body: in an egg undoubtedly this drop beats and throbs. The animal itself is formed out of the white of the egg, but its food is in the yolk. In all cases at the beginning the head is larger than the whole body, and the eyes, which are pressed together, are larger than the head. As the chick grows in size the white turns to the middle and the yolk spreads round it. If on the twentieth day the egg be moved, the voice of the chick already alive is heard inside the shell. At the same time it begins to grow feathers, its posture being such that it has its head above its right foot but its right wing above its head. The yolk gradually disappears. All birds are born feet first, the opposite way to the remaining animals. Some domestic hens lay all their eggs in pairs, and according to Cornelius Celsus occasionally hatch twin chicks, one larger than the other; though some assert that twin chicks are never hatched out. They lay down a rule that the hen should not be required to sit on more than 25 eggs at a time. Hens begin to lay at midwinter, and breed best before the spring equinox: chickens born after midsummer do not attain the proper size, and the later they are hatched the more they fall short of it. [Book 10, 75] It pays best for eggs to be sat on within ten days of laying; older or fresher ones are infertile. An odd number should be put under the hen. If three days after they began to be sat on the top of the eggs held in the tips of the fingers against the light shows a transparent color of a single hue, the eggs are judged to be barren, and others should be substituted for them. They may also be tested in water: an empty egg floats, and consequently people prefer eggs that sink, that is, are full, to put under the hens. But they warn against their being tested by shaking, on the grounds that if the vital veins are displaced the eggs are sterile. The ninth day after a new moon is assigned for starting a hen sitting, as eggs begun earlier do not hatch out. The chicks are hatched more quickly when the days are warm, and consequently eggs will hatch out in 18 days in summer but 24 in winter. If it thunders while the hen is sitting the eggs die, and if she hears the cry of a hawk they go bad. A remedy against thunder is an iron nail placed under the straw in which the eggs lie, or some earth from the plough. In some cases Nature hatches of her own accord even without the hen sitting, as on the dunghills of Egypt. - [Rackham translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Birds 5.58): A hen is a bird, as Augustine says, that is very skillful with her chicks: she gathers them under her wings and caress them, and usually defends and protects them against kites. Pliny: A certain nettle is fatal to chicks by contact. The mothers naturally recognize this plant, and grasping the nettle by the root, they try to pull it out, but they are unable to, because even with the very effort they usually break inside their bodies. Jacobus: The hen's eggs are stolen every day, but they do not stop producing eggs if you leave one. The hen, therefore, carries the eggs from which the males are to hatch, as Pliny says, on the right side of her belly; but the eggs from which the female is to hatch, are on the left side. They eggs that are rounder instead of pointed bring forth the female; but the eggs that are oblong and pointed at the end give birth to the male. The oblong eggs are of a more favorable taste. In all eggs there is a small drop of blood in the vital center, which they think to be the chick;s heart, thinking that it is the first time that they are born. The drop jumps into the eggs and the animal itself palpitates; from every liquid in another egg, something is not incorporated into the chicken. Its food is in the mud. While in the egg, the head is larger than the whole body, and the eyes, when compressed, are larger than the head. But in the the chick growing in the egg the whiteness is turned in the middle, and the yellow is poured around. On the twentieth day the chick moves, and the voice of the living chick is heard within the shell. And from that time it begins to grow feathers, and is in such a position that its head is above its right foot, and its right wing above its head. Meanwhile, the yolk gradually fails. All birds are born on their feet, unlike all other animals. Some hens lay twin eggs and hatch the twins. In Egypt, with quite a wonderful art, eggs are filled and chicks are brought out without fowling. For there the men dig in the sand, and taking dung they wrap the eggs in it, and thus covering the sand, they are warmed by the heat of the sun and the dung, and thus they are filled in due time, and the extracted eggs are split into parts and the chicks come out. We believe that the same thing happens in cold regions, if the eggs are wrapped in warm manure and carefully stored in a mass of very warm manure. As the Experimentator says, There are certain chickens that always produce twins. There is indeed a wonder in the cock, who feeds the hens by calling them, and, while feeding them, compels them to the labors of impregnation and delivery. For, as the Experimentator says, the hen labors a great deal in ovulation; and yet (as John the Philosopher says), she sings after childbirth, and this according to the Gospel: A woman when she gives birth has sorrow, etc. She turns the dust with her feet, and thus obtains sustenance. Laying hens have more sterile eggs than non-laying hens. Many chickens and geese and peacocks lay eggs from the wind, that is, eggs without copulation. But they are small and tasteless, and are more moist than others. When the hen lays in the hot summer, the eggs are cracked by themselves, and the chicks come out sooner than in the cold season. There are certain hens that always produce twins, but one of the twins is small and the other large. And for this reason, because the larger one occupied the most space and the small one lay compressed, so it happens that its members are badly arranged and make a monster. Those born after the solstice do not fill to their proper size, and so much less, the later they come. - [Badke translation/paraphrase]

Bartholomaeus Anglicus [13th century CE] (Liber de proprietatibus rerum, Book12.18): The henne is called Gallina, and hath that name of Gallo, the cocke, as the Lyonnesse hath the name of the Lyon. And as some men meane, if her members were medled with gold when it is molt, the gold should wast, as Isidore sayeth. The Henne is a soule of great laieng and bréeding, and layeth many egges without treading as Aristotle sayth, libro. 5. And they bee called winde Egges, and bée more unsavourye and lesse worthye then other Egges. And some hens have alway twins, two chickens in one shell. And one of the twins is little, and sometime wonderfullye shapen. After that they sit on broode three daies, anone tokens and signes of Chickens bée seene: And the Chicken is bread of the white, and nourished with the yolke, as hee saith. Also hens that laie too much be not of long lyfe, but they die soone, as it is sayd lib. 6. Other properties of hens that bée known nigh to all men, be touched in the Glose super. Mat. 18. There it is sayd yt a hen is a mild bird about chickens: for she covereth chickens under her wings, and defendeth them against the Kite, and taketh sicknesse for sorrowe of her chickens, and looseth her feathers, and féedeth her chickens more then her selfe. And when shée findeth meate, shee clocketh and calleth her chickens together, and to defend her chickens, shée putteth her selves against a stronger then hir selfe. And also shée fighteth with a man for defence of ye Chickins. When the chickins bée dispearpled, shée clocketh and calleth them together, and covereth them under her wing. And defendeth them, that they bée not taken with Hawke, nor with Kite, and her kindlye love about her Chickinnes is knowen by roughnesse of feathers, and by hoarsnesse of voice. - [Batman]