question: How do baby birds breathe inside their eggs?
comment: Directly under the egg's shell are two membranes. When the eggs are laid by the mother they’re very warm, and as they cool the material inside the egg shrinks a little bit. The two membranes pull apart a little and create a small pocket or sack of air. As the developing bird grows, it breathes in oxygen from the air sack and exhales carbon dioxide. Several thousand microscopic pores all over the surface of the egg allow the CO2 to escape and fresh air to get in. Source: URL_0
passages: Something similar occurs with the respiratory system. In mammals, the total inner volume of lungs changes during ventilation. However, this does not happen in birds. They make the air circulate through their lungs by contracting and expanding their air sacs. Five genes are involved in this process in mammals and birds.
   and their appearance around the seventh month marks the point at which limited respiration would be possible, and the premature baby could survive.
  
  Section::::Development.:Vitamin A deficiency.
   At birth, the baby's lungs are filled with fluid secreted by the lungs and are not inflated. After birth the infant's central nervous system reacts to the sudden change in temperature and environment. This triggers the first breath, within about 10 seconds after delivery. Before birth, the lungs are filled with fetal lung fluid. After the first breath, the fluid is quickly absorbed into the body or exhaled. The resistance in the lung's blood vessels decreases giving an increased surface area for gas exchange, and the lungs begin to breathe spontaneously. This accompanies other changes which result in an increased amount of blood entering the lung tissues.
   Song learning in juvenile birds occurs in two stages: sensory learning, which involves the juvenile listening to the father or other conspecific bird and memorizing the spectral and temporal qualities of the song (song template), and sensorimotor learning, which involves the juvenile bird producing its own vocalizations and practicing its song until it accurately matches the memorized song template.
   Air passes unidirectionally through the lungs during both exhalation and inspiration, causing, except for the oxygen-poor dead space air left in the trachea after exhalation and breathed in at the beginning of inhalation, little to no mixing of new oxygen-rich air with spent oxygen-poor air (as occurs in mammalian lungs), changing only (from oxygen-rich to oxygen-poor) as it moves (unidirectionally) through the parabronchi.
   Section::::Concept and analysis.
   After two or three months of song learning and rehearsal (depending on species), the juvenile produces a crystallized song, characterized by spectral and temporal stereotypy (very low variability in syllable production and syllable order). Some birds, such as zebra finches, which are the most popular species for birdsong research, have overlapping sensory and sensorimotor learning stages.
   After the eggs hatch, parents provide varying degrees of care in terms of food and protection. Precocial birds can care for themselves independently within minutes of hatching; altricial hatchlings are helpless, blind, and naked, and require extended parental care. The chicks of many ground-nesting birds such as partridges and waders are often able to run virtually immediately after hatching; such birds are referred to as nidifugous. The young of hole-nesters though, are often totally incapable of unassisted survival. The process whereby a chick acquires feathers until it can fly is called "fledging".
   The lungs of birds are relatively small, but are connected to 8 or 9 air sacs that extend through much of the body, and are in turn connected to air spaces within the bones. On inhalation, air travels through the trachea of a bird into the air sacs. Air then travels continuously from the air sacs at the back, through the lungs, which are relatively fixed in size, to the air sacs at the front. From here, the air is exhaled. These fixed size lungs are called "circulatory lungs", as distinct from the "bellows-type lungs" found in most other animals.
   Section::::Avian blastocoel.
  
  Similar to mammals, fertilization of the avian ovum occurs in the oviduct. From there the blastodisc, a small cluster of cells in the animal pole of the egg, then undergoes discoidal meroblastic cleavage. The blastoderm develops into the epiblast and hypoblast and it is between these layers that the blastocoel will form. The shape and formation of the avian blastodisc differs from amphibian, fish, and echinoderm blastulas, but the overall spatial relationship of the blastocoel remains the same.
  
  Section::::Avian blastocoel.:Formation of primitive streak.
   No embryos are known from "G. major" eggs, but they are usually assumed to have been laid by a similar type of bird.
  
  Section::::Description.:Nests.
   Since no other species of bird have been found that have pouches, it is difficult to say how this evolved; however, in Jacanas, a tropical wading bird found in the same habitat as finfeet in which males also provide most of the parental care, young are frequently sheltered and even carried by tucking them under the wings and holding them against the body. This could have been the ancestral state for Sungrebes, as well.
  
  Section::::Behaviour and ecology.:Food and feeding.
   Section::::Circulation.:Development of the cardiovascular system.
  
  In early stages of development in amphibians, it has been discovered that convectional gas transport, the method through which hemoglobin is carried throughout the body, is an unnecessary process. This is important because it indicates that the coupling of ventilator gas transport and hemoglobin gas transport are independent mechanisms and not yet coupled as they are in adulthood. An example of this difference is seen in the response to hypoxia between organisms at differing developmental stages. In juvenile newts, there is no cardiovascular response in conditions of hypoxia.
   The respiratory system of birds differs significantly from that found in mammals. Firstly, they have rigid lungs which do not expand and contract during the breathing cycle. Instead an extensive system of air sacs (Fig. 15) distributed throughout their bodies act as the bellows drawing environmental air into the sacs, and expelling the spent air after it has passed through the lungs (Fig. 18). Birds also do not have diaphragms or pleural cavities.
   The smothering of eggs with food-grade corn oil is another method employed by authorized persons. Once applied, the eggs' porous surface, which allows the transport of oxygen, becomes clogged with oil. The parent birds continue to incubate the oiled eggs, which ultimately fail to hatch.
  
  Section::::Culling by region.
  
  Cormorant culling has occurred in many countries including Australia, the United Kingdom, Germany, Estonia, Japan, the United States and Canada.
  
  Section::::Culling by region.:Australia.
   Nestlings are equipped with 44 spoon-shaped teeth to feed on the outer layer of their mother's skin. Young feed all at once for some seven minutes; then they all rest for three days as the female grows a new outer skin layer. This phenomenon is known as maternal dermatophagy. This practice and morphological similarities are shared with its African relative "Boulengerula taitana", suggesting it evolved over 100 million years ago.
   Monotremes, only five species of which exist, all from Australia and New Guinea, are mammals that lay eggs. They have one opening for excretion and reproduction called the cloaca. They hold the eggs internally for several weeks, providing nutrients, and then lay them and cover them like birds. Like marsupial "joeys", monotreme "puggles" are larval and fetus-like, as like them they cannot expand their torso due to the presence of epipubic bones, forcing them to produce undeveloped young.
  
  Section::::Reproductive system.:Marsupials.
   Juvenile bichirs have external gills, a very primitive feature that they share with larval amphibians.
  
  Section::::Anatomy and physiology.:Respiration.:Air breathing.
   Section::::History.
   Males transfer the young to their pouches soon after hatching and keep them there, feeding them and cleaning out their waste, until they are able to swim and feed independently. For some period of time after this, the chicks continue to follow their father and perhaps their mother, often riding on their back. It is unknown how involved mothers are in the care of their young after hatching.
  
  Section::::Behaviour and ecology.:Breeding.:Pouch.
   "H. littorale" can breathe both with gills and through its intestines. However, intestinal respiration is not exhibited upon hatching, and the development of the respiratory intestine occurs throughout the juvenile period (up to 32 days old). Newly hatched larvae do not have the ability to breathe air through their intestines; however, it is possible that they absorb oxygen through their skin at this point, before the armor plates have developed. Soon after, between about 12–23 days of age, young have the capacity to breathe air, but their respiratory intestine has not finished developing. The respiratory intestine is well-developed between days 24 and 32.
   In shallow water, "Polypterus" inhales primarily through its spiracle (blowhole). Exhalation is powered by muscles in the torso. During exhalation, the bony scales in the upper chest become indented. When the muscles are relaxed, the bony scales spring back into position, generating negative pressure within the torso, resulting in a rapid intake of air through the spiracle. The air is nearly sufficient to fill the lungs. This is followed by one cycle of buccal (mouth) pumping, which "tops off" the lungs, with the surplus air from the buccal pumping process discharged through the pharynx. According to one hypothesis, Devonian tetrapods may have inhaled in this way.
   All aquatic reptiles breathe air into lungs. The anatomical structure of the lungs is less complex in reptiles than in mammals, with reptiles lacking the very extensive airway tree structure found in mammalian lungs. Gas exchange in reptiles still occurs in alveoli however, reptiles do not possess a diaphragm. Thus, breathing occurs via a change in the volume of the body cavity which is controlled by contraction of intercostal muscles in all reptiles except turtles. In turtles, contraction of specific pairs of flank muscles governs inspiration or expiration.
   Section::::Reptiles.
  
  The anatomical structure of the lungs is less complex in reptiles than in mammals, with reptiles lacking the very extensive airway tree structure found in mammalian lungs. Gas exchange in reptiles still occurs in alveoli however. Reptiles do not possess a diaphragm. Thus, breathing occurs via a change in the volume of the body cavity which is controlled by contraction of intercostal muscles in all reptiles except turtles. In turtles, contraction of specific pairs of flank muscles governs inhalation and exhalation.
  
  Section::::Amphibians.
   Section::::Special situations.:Children.
  
  There are significant differences in airway anatomy and respiratory physiology between children and adults, and these are taken into careful consideration before performing tracheal intubation of any pediatric patient. The differences, which are quite significant in infants, gradually disappear as the human body approaches a mature age and body mass index.
  
raw_presuppositions: There is no oxygen inside of eggs.
raw_corrections: There is oxygen inside of eggs from the air sack.