Shifting in the duplication time of sex chromosomes in the rat

Duplication of sex chromosomes was studied in bone marrow cells from adult rats and in short-term cultures of rat fetus cells. Results obtained indicate that: a) G₂ period takes 4 hours in cells from fetuses and 3 hours in bone marrow cells, b) S period lasts 7 hours in bone marrow cells and about 20 hours in short-term cultured cells from fetuses, c) In cells from female fetuses one X-chromosome is the last to start DNA synthesis and also one of the X's is the last to end replication, d) In cells from female adult rats both X-chromosomes start and finish DNA synthesis early, e) In both line of cells the Y-chromosome was the last to begin and the last to finish DNA synthesis. — Causes which can account for the differences between the two line of cells are discussed.