METHODS

In a preliminary experiment, three young adult white male mice (Carworth Farms) weighing 13-14 gm were given a subcutaneous injection of 7.5 vc of thymidine-H3 (118 mc/mM) per gm body weight at 1O: OO AM and were killed 8, 24 and 72 hours later, respectively. In the main experiment, a group of 22 male Sherman albino rats, with an average body weight of about 200 gm (table 1) received a subcutaneous injection of 1 I. IC of thymidine-H3 (360 mc/mM) per gm body weight. The injection was given at some time between 8: 00 AM and 1: 00 PM and the animals were sacrificed, one or two at a time, at 1, 3, 6, 12, 24 hours and 3, 6, 9, 18, 27, 42, 60, and 95 days after injection. Later, 4 similar rats were treated in the same manner and sacrificed two at 20 minutes and two at two days after injection.

Tissues removed at autopsy were fixed in Bouin, embedded in paraffin and sectioned at 6 [A. Hematoxylin-eosin-stained sections were covered with NTB3 emulsion by the coating technique of radioautography according to Messier and Leblond ('57). Exposure time varied from 20 days to 6 months.