The difficulty of correlating the microscopic appearances of the lungs in cases of stillbirth and early postnatal death with the results of hydrostatic tests in these cases is stressed. 2. Histologic criteria which depend on the presence of alveoli as evidence of extrauterine respiration are criticized. 3. The view is submitted that the alveolar pattern is the result of a normal developmental process occurring in utero, possibly accentuated by intrauterine fetal respiratory movements, but independent of extrauterine respiration. 4. The glandlike appearance of the lung which has been illustrated in standard textbooks as an example of unrespired fetal lung is probably seen only in relatively early embryos in whom the problem of respiration is unlikely to arise. 5. At or near term the alveolar pattern in the human fetus has developed, certainly in parts of the lung, to such a degree in utero that it cannot readily be distinguished from the pattern seen in the newly-born infant who has breathed for a short time in an extrauterine environment. 6. This view that the alveolar pattern is already fairly well developed at full-term makes for a more intelligible appreciation of the structural appearances seen in the immediately postnatal lung as well as of the physiology of respiration with the first extrauterine breath. 7. The views outlined for the development of the human lung in utero are completely corroborated by experimental studies on pig embryos, intraocular lung implants and the structure of sequestrated lung tissue unconnected with the respiratory tract. 8. Microscopic examination of the lungs may assist in determining fetal age. 9. The medicolegal significance of these observations is discussed.
