The C3H/HeN mice have been bred by brother-sister mating in our laboratory, for more than 20 generations, since they were obtained from the National Cancer Institute, U.S.A. in April, 1956. This report deals with the breeding records and pathology of mammary tumors in these mice.
Of the 333 breeding females, 240 mice (72.0%) developed mammary tumors at an average age of 8.3 months (Table 2 and Fig. 1) . In the virgin females, however, mammary tumors were found in 16 of the 101 mice (15.8%) at an average age of 15.8 months. Survival time of the mice bearing mammary tumors was about 50 daye (Fig. 2) . Histologically, 43 of the 78 primary tumors (55%) showed type A of Dunn's criteria and others type B. No metastasis was found in any tissues other than the lungs and lymph nodes (Table 6) . The pulmonary metastases were fonud in 40% of both breeders and virgin females. Most of the pulmonary metastases appeared to grow intravascularly, but not perivascularly (Photo 3) . Histological findings of the metastases closely resembled the primary tumors.
The C3H/HeN mice bred in our laboratory showed consistently a high susceptibility to the ascites hepatomas MH134, MH129P, MH129F and the ascites mammary tumor FM3A which origi nated in this strain of mouse.
The authors proposed to designate the new substrain of C3H/HeN mice as C3H/HeNSa strain. “Sa” represents the initial of Sendai where our Institute is located.

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Functional properties of the skeletal muscles in mice were investigated by means of EMG. The action potentials in the first discharges and/or repetitive discharges of a single nerves muscular unit (a single NMU) in the fore and hind limb muscles were obtained by a concentric needle electrode. The muscle used were M. rhomboides, M. serratus ventralis, Mm. pectralis, and M. sternomastoideus in the fore limb, M. glutaeus medius, M. quadriceps femoris, M. biceps femoris, M. semitendinosus and M. semimembranosus in the hind limb. The results are summarized as follows:
1) The mono-, di-, and triphasic types of waves were similar to the normal types of NMUs in the other species of animals. Their amplitudes were usually lower than those of the other larger animals and measured 0.2 to 0.8 mV. The short durations of 2 to 7 m sec were characteristic in mice.
2) The range of discharge intervals of a singe NMU distributed between 10 and 80 m sec, and repetition of the discharge was remarkably more frequent than that of the other animals.
The _??_-S curve obtained from a set of _??_-S points shows almost a linear type as observed in fowl, rabbit ahdgoatin character. The _??_-S points obtained from fore limb muscles were distributed in the left side separately from those from hind limb muscles.
3) Nomura's method was also applicable to EMG in mice for analysing the time series of discharge intervals of a NMU. A predominant regulation by the spinal components of the central nervous septum was presumed in the muscle activities of mice because of significant negativity of the first and/or second item of correlogram, downward shift of the S curve, mean interval correlation chart of H type fluctuation and so forth.

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