Cell kinetic studies of the proliferation of the neural epithelium during the embryonic development of the rat brain are described.Pregnant rats received a single injection of "-thymidine PH-TdR) between the tenth and twenty-first day of pregnancy. Autoradiographs of the brains of the 25-day-old offspring were prepared. For a number of different cell types (table 1 , 11) the percentage of labeled neurons as a function of time of prenatal :'H-TdR injection was determined.The end of the proliferating period of the neural epithelial cells of certain cell types can be derived from these percentages of labeled cells (table 1 , V). Furthermore, i t can be demonstrated that the percentage of labeled neurons is equal to the labeling index of the neural epithelial cells of these cell types at the time of prenatal "H-TdR injection.Based on the labeling index of the neural epithelial cells and their S phase, cycle times can be calculated. This way it is possible for the first time to determine the cycle times of those neural epithelial cells that later on differentiate into a certain type of neuron (table 1, IV).The calculated cycle times show that up to the sixteenth prenatal day the neural epithelial cells behave like a homogeneous cell population which proliferates with an approximately constant cycle time of about half a day. The cycle time seems to increase with increasing fetal age.From the total number of a certain type of neuron the number of mitotic divisions or cycle times respectively can be derived which is necessary to produce the number of neural epithelial cells providing these neurons. With regard to the mean cycle time of the neural epithelial cells i t can be concluded that the beginning of proliferation of the neural epithelial cells coincides with the formation of the neural plate on the ninth day of embryonic development or starts even shortly before this time.Prenatal labeling with :3H-thymidine ("H-TdR) of the proliferating neural epithelial cells of the embryonic brain and the observation of labeled neurons in the young or adult animal has proved a useful tool in studying the development of the brain. Labeling of neurons can only be achieved if :jH-TdR is applied prenatally, prior to differentiation of the neural epithelial cells into neuroblasts (cells produced by the last division of neural epithelial cells that do not divide any more but differentiate into the corresponding neurons) and neurons. With this method the prenatal "time of origin" or "date of birth" of many types of neurons in the chick, the mouse and the rat has been studied.

209Bi nuclear magnetic resonance (NMR) was studied in half-Heusler compounds YPdBi and YPtBi. The observed NMR spectra exhibit low intensity because of short nuclear transverse relaxation time (T 2) governed by sizable structural disorder. In both compounds, the frequency shift of the 209Bi bulk nuclei is dominated by the chemical shielding (δcs) contribution. Remarkably, it is positive for topologically trivial YPdBi yet strongly negative for topologically nontrivial YPtBi. At the same time, the spin–lattice relaxation rate (1/T 1) is distinctly larger in YPtBi due to stronger spin–orbit coupling and postulated inversion of the electronic bands near the Fermi level. The unique features of δcs and 1/T 1 established for YPtBi can be considered as possibly universal fingerprints of topologically nontrivial state in similar materials.

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209Bi nuclear magnetic resonance (NMR) was measured in half-Heusler bismuthides ScPdBi, LuPdBi, and LuPtBi and discussed in conjunction with the NMR data recently reported for YPdBi and YPtBi. The observed 209Bi NMR spectra exhibit low intensity because of short nuclear transverse relaxation time (T 2) governed by sizable structural disorder. Remarkably, the NMR frequency shifts are positive for topologically trivial ScPdBi and YPdBi, yet strongly negative for topologically nontrivial YPtBi, LuPtBi, and LuPdBi with postulated inversion of the electronic bands near the Fermi level. For the former two compounds, the spin–lattice relaxation rate (1/T 1) is very large and seems correlated with the strength of spin–orbit coupling. The study demonstrates the capability of NMR spectroscopy to probe the band inversion in putative topological insulators.