There is relatively little published information on the embryonic formation of the diaphragm, compared to the amount of developmental information available on other structures. It is known, however, that the diaphragm is composed of two muscles that originate from four separate structures. The costa and the crurae are innervated from different sources in addition to being derived from different structures. The diaphragm exhibits two forms of patterned activity in utero, one of which evolves into the stereotyped pattern of breathing that is present once the neonate takes its first breath. Much research has been done on the postnatal development of the diaphragm, especially on its changing muscle fiber composition. The different muscle fiber compostition and therefore different MHC expression of the fetus, the neonate, and adult diaphragm has been implictated in the differential fatiguability of neonates and adults. It has yet to be resolved whether the neonate diaphragm is more or less fatigue-resistance than the adult diaphragm; the many studies on this subject conflict each other.