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A yolkless egg is most often a pullet's first egg, produced before her laying mechanism is fully ready. In a mature hen, a yolkless egg is unlikely, but can occur if a bit of reproductive tissue breaks away, stimulating the egg-producing glands to treat it as a yolk and wrap it in albumen, membranes and a shell as it travels through the egg tube.
Illustration of a woman collecting hens' eggs from Tacuina sanitatis, 14th century. The chicken or the egg causality dilemma is commonly stated as the question, "which came first: the chicken or the egg?" The dilemma stems from the observation that all chickens hatch from eggs and all chicken eggs are laid by chickens.
The traditional modes of reproduction include oviparity, taken to be the ancestral condition, traditionally where either unfertilised oocytes or fertilised eggs are spawned, and viviparity traditionally including any mechanism where young are born live, or where the development of the young is supported by either parent in or on any part of their body.
The UK alone consumes more than 34 million eggs per day. [84] Hens of some breeds can produce over 300 eggs per year; the highest authenticated rate of egg laying is 371 eggs in 364 days. [85] After 12 months of laying, the commercial hen's egg-laying ability declines to the point where the flock is commercially unviable.
Because male chickens do not lay eggs and only those in breeding programmes are required to fertilise eggs, they are considered redundant to the egg-laying industry and are usually killed shortly after being sexed, which occurs just days after they are conceived or after they hatch. [3]
The only living mammals that lay eggs are echidnas and platypuses. In the latter, the eggs develop in utero for about 28 days, with only about 10 days of external incubation (in contrast to a chicken egg, which spends about one day in tract and 21 days externally). [11] After laying her eggs, the female curls around them.
Typically large numbers of eggs are laid at one time (an adult female cod can produce 4–6 million eggs in one spawning) and the eggs are then left to develop without parental care. When the larvae hatch from the egg, they often carry the remains of the yolk in a yolk sac which continues to nourish the larvae for a few days as they learn how ...
Older hens gradually produce fewer eggs, and the eggs are usually larger. [1] Since the average lifespan of a pet layer hen is 8–15 years, [2] henopause has received attention as a potential problem for backyard or urban chicken farmers who are eventually faced with the decision to either slaughter older layers or keep them as non-producing pets.