Chick embryos ranging i n age from 18 to 32 hours were treated with thymidine-H3 to examine the cell cycle and nuclear migration of the neuro-epithelial cells during closure of the neural tube.By plotting the percentage of labeled metaphases against the time of thymidine-H:' application, the DNA-synthetic period was found to last five hours, the postduplication (G2) stage and prophase two and one-half hours and the preduplication ( G I ) stage to be brief or non-existent. The total cell cycle time was calculated to be eight hours. During DNA synthesis the nuclei are located near the basement membrane; subsequently they migrate to the lumen to undergo mitosis, after which they return to the outer zone to start DNA synthesis for the next cell division.To determine the life span of the neuro-epithelial cells by a different method, chick embryos were treated with vincristine, a drug known to arrest the mitosis in the metaphase. Fifteen, 26, 34 and 41% of the neuro-epithelial cells were arrested in the metaphase, one. two, three and four hours after treatment, respectively. Since the "mitotic index" of the neuro-epithelial cells was 5.2, the duration of the mitosis was calculated to last 25 minutes, and the life span of the neuro-epithelial cell to be eight hours. Immediately after closure of the neural tube, a different cell type characterized by a round nucleus, pale nucleoplasm and a dark-staining nucleolus appeared at the periphery of the tube near the basement membrane. These cells, which originate by division from the neuro-epithelial cells at the lumen, did not incorporate thymidine-H.' at any time during further development. They are considered to be primitive neuroblasts and to form the first representatives of the mantle layer.

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