[Regional shifts in the vertebral column of inbred mice treated with X-rays or FCdR].

In three inbred-strains of mice (C57 BL/6, C57 BL/10 and C57 BL/Ks) 5-fluoro2'-deoxycytidine (FCdR) and X-rays have been applied at several stages of pregnancy. The shiftings of the regions of the vertebral column in the offspring have been examined. 1. There are several critical periods concerning differentiation of the cervical ribs. 2. Critical periods for differentiation of lumbar ribs are at stages day VIII and day IX of pregnancy. 3. X-rays and FCdR, applied at stage day IX, inhibit the formation of cervical ribs; in the controls they were found in a high percentage rate. 4. The appearance of the 13th ribs is inhibited by X-rays at stage day X+20 h of pregnancy. 5. The appearance of the 13th ribs is inhibited by FCdR at stage day IX; lumbar ribs have their critical period at the same time. 6. First ribs were never absent in the control and experimental series. 7. At stage day IX+20 h X-rays induced at a third time cervical ribs. This shows the importance of a broadly timed experiment. 8. The border between the lumbar and the sacral region is very sensitive to exogeneous influences; in the controls asymmetrical vertebrae fixed at the pelvic girdle are rather frequent.