[Variability in developmental stage of mouse embryos (C 5 7 BL) in the early period of organogenesis].

Individual differences in developmental stages of mouse embryos of the two related strains C57 BL/6JHanFfm and C57BL/10JFfm during the period between day VIII and day X p.c. have been examined.The following results were obtained: 1. A remarkable variability in the number of somites could be stated in embryos of different litters but also in embryos of the same litter, especially at stages day IX and day IX +12 h. 2. There was a close relationship between the number of somites and progresses of rotation from the S- to the C-shape. Incidence of pre-rotation stages was higher in embryos of strain C57BL/6 than in embryos of strain C57BL/10. 3. Individual differences were stated in the developmental stages of neural groove and upper limb buds. The degree of differentiation and closure of the neural groove was more progressive in strain C57BL/10. 4. Individual differences were also registered in the appearance and development of the following primordia: eye, ear, heart, thyroid, lung, liver, ureter and the pronephric ducts. 5. The body length of the embryos was closely related to the number of somites.