Two distinct forms of the gonocyte, or primitive germ cell, were delimited in the testis of the prepuberal bull. The most primitive of these, the type I gonocyte, gives rise through its mitoses to the type II gonocyte. Type II gonocytes were highly suceptible to the necrotizing effects of ionizing radiation. A dose of 19 rads killed 50% of their number and the residue was all but obliterated by a dose of 38 rads. The${\rm D}_{37}$ for these cells was estimated to be 25 rads. Conversely, the type I gonocyte was highly refractory to irradiation. Some of them survived a dose of 220 rads-a dose lethal to the calf. Gonocytic survival was unaffected by dose rate (0.12-28 rads/min). A dose of 38 rads or more will permanently impair the sperm-producing capacity of bulls irradiated between the 80th day of prenatal life and the 70th day of postnatal life with the magnitude of the effect diminishing as age at irradiation increases. Following exposures sublethal to the bull, increases in both numbers and activity of type I gonocytes plus the appearance of type A spermatogonia in Day 70 postnatal testis are believed to account for the diminution and eventual loss of a permanent testicular effect.

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