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1. An investigation was made of the sequence of changes that occurred during the development of vitamin A deficiency in the chick. Particular attention was given to changes in the central nervous system (CNS) and in bone.
2. Groups of birds were killed after being fed the diet for 14, 17, 20, 22, 24 and 28 days. The tissues of five deficient and five control birds were examined unfixed for gross changes and photographed. Groups of five deficient and five control chicks were perfused through the heart with Heidenhainαs Susa solution and, after decalcification, the tissues were examined for gross and histological changes. The bones of other groups of five deficient and five control chicks were examined for tetracycline-induced fluorescence.
3. The first lesion seen was metaplasia of the nasal respiratory epithelium to a stratified squamous form. This was rapidly followed be decreased weight gain, the development of ataxia, compression of the CNS and Wallerian degeneration in the ventral and lateral columns of the cervical spinal cord.
4. Changes in bone were first seen in chicks killed 6 days after metaplasia of nasal respiratory epithelium and 3 days after ataxia and brain compression had been seen.
5. Changes were present in the epiphyseal cartilage zone of the femur and in the periosteum of the vertebrae, but not in the epiphyseal cartilage of the vertebrae. the changes in the femur were thought not to have a direct relationship to vitamin A deficiency and may have been produced by inanition.
6. An increase in osteoblast activity and cartilage formation was seen in the vertebral periosteum of the vitamin A-deficient chicks and the relationship of these changes to lesions in the CNS is discussed.