Abstract
1. 1. Candling methods were refined that resulted in 97 per cent of eggs opened containing chick embryos at the desired stage 19. 2. 2. Specific activity of alkaline phosphatase (activity per mg N) was determined biochemically for whole chick brains from stage 16 to stage 20. Activity declined during this period, corroborating earlier histochemical observations. 3. 3. The “cytoplasmic” fraction of stage 19 brains (contaminated with some nuclei) contains 100 per cent of the expected specific activity of alkaline phosphatase for whole, unfractionated brain tissue. 4. 4. The “nuclear” fraction of stage 19 brains (estimated 90 per cent nuclei and 10 per cent cytoplasm) contains 65 per cent of the expected specific activity of alkaline phosphatase of whole, unfractionated brain tissue. 5. 5. Phosphatase activities of the various centrifugation supernatants indicate that leaching of phosphatase from nuclei is probably not the explanation for the observed lower activity of the nuclei. 6. 6. A “nuclear” fraction in which nuclei are broken and dispersed as small particles by the buffer used in the biochemical determinations has a lower specific activity than one which is completely rehomogenized before assay, indicating that soluble phosphatase is not preferentially adsorbed on nuclear surfaces. 7. 7. Gomori method histochemical staining of fractionated tissue stains isolated cytoplasm black, nuclei blacker than cytoplasm only when surrounded by cytoplasm, and stains nuclei little if at all when they lie free of cytoplasm in small numbers. 8. 8. Results are interpreted as showing that nuclei in this tissue contain decidedly less alkaline phosphatase than cytoplasm, contrary to histochemical results on whole tissue. Results relative to the question as to whether nuclei in this tissue at this development time have any phosphatase of their own at all, remain equivocal. 9. 9. The most obvious correlation with the alkaline phosphatase activity curve for these early stages of brain development is the curve for mitotic index of the same tissue.
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