Abstract
In embryos of ddN-and CF#1-mouse and Wistar rat, the formation of some organs were observed during a period of organogenesis, namely, from the 7th or 8th day to the 13th day of pregnancy. Young embryos at various ages were taken from several mothers and were fixed in Bouin's fixative, then were sectioned through paraffin and stained with hematoxylin and eosin. In CF#1-mouse, somites in early stages as well as a number of organs derived from a central nervous system developed more rapidly under a difference corresponding to developmental process during 1/2 days in ddN-mouse. Since the 11th day of pregnancy, there was no difference in the number of somites between both strains of mouse. Wistar rat displayed about one day's retardation of development corresponding to proceedings of organogenesis in the mouse. Meanwhile, in spite of a parallel increase of somites in both strains of mouse since 11th day, the formation of some mesodermal organs such as sclerotome and eye-lid was earlier in CF#1 mouse than ddN-mouse. The correlation between an increase of somites and a period of the beginning of organogenesis was not always the same in each strain of mouse. The correlation in the rat was similar to in CF # 1-mouse. Fluctuations in a period of the beginning of organogenesis due to the difference of strain was observed in respective strains used. The fluctuations in normal developmental stages could be called "The strain-difference". Therefore, it would be undesirable that the developmental stages were expressed with only embryonic ages or with only the number of somites. Hitherto, the fluctuation of results in teratological or other experiments have been explained with a simple word, "The strain-difference". But, when fluctuations of developmental stages due to the difference of strain were considered, it would be difficult to distinguish between the divergence based on "The true strain-difference" and the divergence based on difference in proceedings of the normal development. Hereafter further studies to relate to "The strain-difference" in the process of the normal development of experimental animals have to be carried out.
