Nomarski interference contrast views of the hindbrains of 1-week-old irradiated zebra fish larvae. The animals were irradiated at 6 hr after fertilization using a cesium source in a Model M Gammator (Radiation International). The nominal dose rate was 980 rad/min, and the embryos were exposed for 1.0 min. After continued development the animals were examined, as living larvae, by microscopy (Kimmel, 1972). (A) Animal shown with the normal pair of M-axons (M) which accompany the medial longitudinal fascicle (mlf) of finer axons through the hindbrain. (B) Both M-axons are absent, and only the mlf is present. In scoring for missing M-cells, the presence of this fiber pathway was always ascertained to provide an internal control for visibility. Dimension marker = 10 μm.

Autoradiographs of labeled (A) and unlabeled (B) M-cells after injections of [³H]thymidine. The yolk sacs of embryos were injected 9 (A) or 15 hr (B) after fertilization (25.5°C) with about 5 nl of isotope ([methyl-³H]thymidine, 1.0 mCi/ml, 6.7 Ci/mmole, New England Nuclear). After development for 1 week, the animals were fixed overnight with an aldehyde mixture [4% formaldehyde, 2% glutaraldehyde, 8 mM CaCl₂, 0.15 M cacodylate-HCl (pH 7.0)]. They were washed thoroughly, dehydrated through ethanols, and embedded in Epon-Araldite. Serial 5-μm transverse sections were taken and coated with Kodak NTB 2 emulsion. After exposure times of about 9 days, the slides were developed and stained with 1% toluidine blue. In all cases neurons near the M-cells showed specific labeling over nuclei demonstrating that the label was available and that incorporation was specific. Dimension marker = 10 μm.