[Fate maps and determination in the female genital and anal primordia of the housefly].

1. The genital and anal apparatus of the female housefly (Fig. 1) develops fromthree imaginai disks: The median genital disk furnishes the hind gut, the cuticular anal appendages and the parovaria; the pair of lateral genital disks combines to form the outer parts of the 8th abdominal segment and the internal genital apparatus, except for the parovaria and the gonads (Fig. 3). 2. Thefate maps of the median (Fig. 5) and lateral genital disks (Fig. 8) were established by transplantation of genetically marked disk fragments. 3. Symmetrically halvedmedian genital disks, even after transplantation into metamorphosing hosts, produced more bristles within the ventral anal plate area than would be expected from their prospective significance (Fig. 6b). 4. If metamorphosis is delayed by transplanting halves of median genital disks into younger larval hosts (Fig. 6c) or by culturing them in adult females (Fig. 6d), symmetrical reduplication may occur. 5. Upon transplantation oflateral genital disks, partial (spermathecae) or complete (copulatory vesicles) regulation was observed even if metamorphosis had not been delayed (Fig. 3). This cannot be due to injury of the disks, since uninjured disks developingin situ regulate in the same way, if the median genital disk has been extirpated (Fig. 11). 6. After being cultured for four days in younger larvae, a single lateral genital disk can differentiate the full complement of structures normally produced by apair of such disks (Fig. 9b). 7. Transdetermination was observed in three cases: When transplanted from late to early third instar larvae, lateral genital disks produced spermatheca capsules with allotypic bristles (Fig. 12). 8. Thecell lineage in the primordia of the female genital and anal apparatus was investigated by analyzing genetically marked clones. These were produced by irradiating with X-rays 2nd instar larvae heterozygous for thebrownbody gene. In this stage, tergal and sternal elements are not yet determined in either the median or the lateral genital disks (Fig. 14). 9. The approximatecell numbers of the postabdominal anlagen for cuticular parts in mid 2nd instar larvae were estimated from the average size of the mosaic spots. The median genital disk was found to contain about 9 cells and the lateral disks 2-3 cells each. 10. The results provide new information on the comparative embryology of the dipterans (Fig. 15). 11. The phenomenon of transdetermination in imaginai disks is discussed with regard to the state of determination and the extent of proliferation.