It has been often found that vertebral numbers of mice are varied among a few inbred strains as well as shapes of bones. This paper describes the distribution of vertebral numbers of inbred mice-strains dd, rr, ss, and C57BL, during 60 days after delivery to compare the mode of vertebral development among them.
1) Numbers of cervical and thoracal vertebrae were 7 and 13 respectively in all mice of 4 inbred strains regardless of ages.
2) In most of the cases lumbar vertebral numbers were 6 in dd, ss, and C57BL, but 5 in rr. Skeletal types of asymmetry, caused by the sacralization of the last lumbar vertebra either in the right or in the left side, was found more frequently in dd and rr than in the others, and to be the least in ss. These asymmetrical types were observed in higher frequency in the right side except the case of ss strain.
According to this sacralization of the lumbar vertebra, distribution of sacral vertebral types in each strain showed the same tendency as that of lumbar vertebral numbers. Sacral vertebral numbers, however, were mostly 4 or 3, and showed the similar manner of distribution in dd, rr, and C57BL, but not in ss strain.
3) Caudal vertebral numbers appeared to increase rapidly until 10 days following the delivery in each strain, thus attained to those of adult mice. But it took roughly 30 days until both shapes and numbers became same as adults'. As for the caudal vertebral numbers, there was no difference among the strains.
4) Variation of lumbar and sacral vertebral numbers was marked in each strain. This seems to be due to the reason that the lumbar region begins to develop later than the other regions, and the environment is ready to influence the development of vertebrae in that region at early embryonic stage.
5) Interfrontal bone was found most frequently in C57BL, and was less in decreasing order of rr, dd and ss. Mice having this bone are not considered to be abnormal in any pathological sense, because of their normal performance of growth and of reproduction.

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The requirements for the successful artificial insemination with mice and rats were investigated in following points; an induction of pseudopregnancy, determination of the optimum time of insemination, extraction of semen, diluent for semen, and the technique of semen injection.
Physiological salt solution, Rock solution and glucose-phosphate-buffer solution were compared as a diluent of sperms. The Rock solution was found to be the best among above three sorts of diluent in regard to the survival and vitability of sperms.
The artificial insemination was performed with 16 mice employing thus established techniques. Four out of 10 mice artificially inseminated at night 9 to 12 p. m. underwent normal delivery, while in contrast to this, not even a single trial ensued successful with 6 females inseminated during daytime through night earlier than 8 p.m. So far in a single rat, inseminated at 9 p.m., pregnancy and delivery proceeded normally.
In the artificial insemination either in mice or in rats at night, the rate of the successful performance was apparently higher when females were injected with the sperm suspension at the same time as natural mating.

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