1. Six groups of mice of the ddN-strain were exposed to a single, whole body X-radiation of 200 r. on the different days between the 8th and the 13th day of pregnancy. Others were exposed to 150 r. or 300 r. on the 12th day of pregnancy. Mice of the CF1strain were also exposed to 200 r. on the 12th day of pregnancy.

    Embryonic and foetal digits were examined both externally and microscopically 2, 6, 12, 18, 24, 48 and 72 hr. after X-radiation. After that they were examined at 1-day intervals until the 19th day of foetal age. Their ossified skeleton was studied by the Dawson method and their cartilaginous one was studied by Noback’s modification of van Wijhe’s method.

  2. The critical stage for digital malformations in the ddN-mouse was presumed to be in the beginning of the 12th day of embryonic stage. Digital rays with precartilage concentrations of the mesenchyme became obvious at this period.

  3. The incidence of digital malformations was proportional to the X-ray dose employed. Most malformations were of the reduction type, i.e. partial or complete ectrodactylism.

  4. The number of digits missing was proportional to the X-ray dose. The 2nd and the 3rd digits were most susceptible, while the 4th digit was most resistant. It was noticed that each malformation manifested itself also in various transitional forms.

  5. Differences between the ddN- and the CF1-strain in their tendency to manifest digital abnormalities were observed.

  6. Localized foci of cellular necrosis were already evident in the primordia of the digital bud 2 hr. after X-radiation and the pyknotic nuclei were almost eliminated after 24 hr.

  7. The prospective pattern of malformations could already be detected externally before the cleft between the digits became marked.

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