1. X-rays in doses of from 18 H to 55 H cause the breakage of a certain proportion of double X-chromosomes in the germ cells of Drosophila melanogaster.

2. This breakage is of a different sort from that which occurs sporadically in individuals unexposed to X-rays.

3. The breakage due to X-rays occurs in cells exposed at or near the maturation period, being evident in individuals developing from eggs laid three to six days after exposure, but little or not at all in individuals from eggs laid several days later.

4. The breakage occurs in approximately 3 per cent. of cells in the susceptible stage of development, when doses of 36 to 55 H are used.

5. The genetic evidence indicates that the breakage due to X-rays probably occurs in most cases at or near the junction the two arms of the double-X.

6. The effect is probably caused indirectly, by the rays changing the general conditions in the nucleus, rather than by a localised action of primary or secondary rays in striking the locus to be affected.

7. The breakage does not seem to be a result of any abnormal crossing over that might have been caused by the X-rays.

8. Although a similar breakage may be caused by X-rays in the long autosomes, it probably would not be detected owing to inviability of the resulting individuals.

9. The "natural" breakage of the double-X which occurs sporadically, independently of exposure to X-rays, takes place not only near the maturation period, but more often in earlier oögonia.

10. The genetic evidence indicates that this "natural" breakage may occur at any point along the chromosome, and that, when the shorter arm of the resulting J-shaped chromosome exceeds a certain size, the zygotes ("females") containing the J and, in addition, a single X-chromosome are inviable.

11. Stocks differ genetically in the strength of their tendency to "natural" breakage.

12. Certain influences, at present obscure, which vary from one experiment to another, affect considerably the viability of otherwise diploid individuals containing three X-chromosomes (Bridges' "superfemales").

13. Only negative evidence was obtained regarding the question of the possible production of deficiencies and other mutations by X-rays, either in the sex chromosomes or the third chromosome.

14. Some evidence, however, points to the conclusion that X-rays tend to cause non-disjunction, not only of the X-chromosomes, but also of the fourth and other autosomes.

15. Corroboration was incidentally obtained, in a count of over 35,000 flies, of the-theory that reverse "mutation" of bar eye does not occur in the male, where there is no opportunity for crossing over.

16. When "natural" breakage occurs in oöginia, cells are formed having the male genetic composition (XY type), but these cells, or their descendant cells having the same composition, nevertheless form eggs. The change to male composition occurred, in two instances, about ten cell divisions antecedent to the oöcyte stage, so that any hypothetical "female substances" or "egg-forming substances" originally present would have been diluted approximately 1000 times before egg formation commenced.

17. It therefore becomes probable that oögenesis in Drosophila is brought about by a chain of reactions commencing many cell-generations previous to the oöcyte stage, and that the sex-determining genes do not intervene differentially in this reaction-chain after this early period. Their rôle in differentiating oögenesis from spermatogenesis is hence confined to deciding, long before the visible manifestations characteristic of gametogenesis appear, whether the egg or the sperm reaction-chain shall be started. Their action is in this respect similar to that of the influences determining the development of the amphibian limb and other organs which, after an early period of invisible preparation, become "self-differentiating".

Department of Zoology contribution No. 196.