3.5. Embryonic Development
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Stages 1 and 2
Stage 1: 4 - 5 weeks after fertilisation
The laryngotracheal tube forms from a groove that pushes out of the gastrointestinal tract (specifically, it pushes out of the pharynx).
Stage 2: 5 - 17 weeks after fertilisation
Lung buds develop from the laryngotracheal tube and invaginate (push into) the wall of the intraembryonic coelom, which is one large single cavity. The intraembryonic coelom will divide up during development to form various cavities of the body, i.e. the pericardial cavity, peritoneal cavity and the bilateral pleural cavities. The lung buds invaginate further and form the primary bronchi, bronchioles and eventually alveoli. The intraembryonic coelom is gradually pushed flat by the developing lung buds.
Stage 3: 40+ weeks after fertilisation
The visceral and parietal pleural layers can now be seen as different layers. These pleural layers used to be the inner and outer walls of the intraembryonic coelom. The pleural cavities can also be seen. This used to be called the intraembryonic coelomic cavity.