Influence of Yolk on Mitotic Rate in Untreated and X-Rayed Grasshopper Neuroblasts in vitro.

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Abstract
Neuroblasts of the Chortophaga viridifasciata embryo were studied in hanging-drop cultures. Frequency of mid-mitotic neuroblasts was found to increase as the quantity of yolk in the culture was increased up to a quantity equivalent to 1/4 that in an egg. The influence of yolk on mitosis in the untreated neuroblast is interpreted as indicating that, in this cell at least, the first half of prophase does not operate solely on energy stored within the cell but is dependent on an outside source of energy for continuance of mitosis. In X-rayed embryos it was found that the duration of the radiation-induced mitotic inhibition is shorter and the beginning and completion of recovery is faster in the neuroblasts of embryos cultured in yolk than in those of embryos cultured without yolk. The pronounced positive effect of yolk on recovery of neuroblasts from radiation damage demonstrates that, in this cell, some extracellular substance or substances can greatly enhance repair of mitotic damage.