The author has already reported on the subject entitled "The life history of the Japanese mouse, Mus molossinus Temminck and Schlegel I-XI", that is, 1) on the devices of rearing, 2) estrous cycle, copulation, pregnancy and parturition in the rearing cages, 3) litter size, number of the weanling per litter and sex ratio, 4) postnatal development of the sucklings and growth after birth, 5) on the onset of puberty, decline of reproductive function and life span, 6) notes on the breeding habit, 7) some notes on the sexual cycle, 8) habitat of the mouse and its reproduction, 9) prenatal growth of body weight, neck- and crown-rump length, length of tail and hind foot, and size of genital organs in the fetus, 10) postnatal growth of head and body, tail, hind foot, ear, skull and gonads, 11) Measurements of external characters, skull and reproductive organs in adult mouse. In the present paper I have reported on the gametogenesis and early development as the last one of a series of the study. The nuclei of the oogonia have reached the pachytene stage by the time of birth. By 5 days post partum all the oocytes in the ovary have a single layer of follicular cells, growing approximately three or four times of the oogonia in size. By 10 days post partum the oocytes are two times of the oocytes of 5 days post partum in size. At 20 days post partum some of the follicles acquire for the first time a small antrum. The first estrus takes place at 50 to 60 days post partum, and at that time the ovum is surrounded by the cells of the cumulus oophorus in the antrum of the mature follicle. Ovulation occurs soon after this condition is established. The seminiferous tubules in the new-born mouse have primordial germ cells and spermatogonia. But the primordial germ cells disappeared at about 5 to 10 days post partum. At 15 days post partum appear spermatocytes with the pachytene nuclei. Each of them contains a large deepstaining nuclear structure which was not present in the spermatogonia at any time. The lumen appears first in some of the seminiferous tubules at 15 days post partum. After this time spermatogenesis appears to proceed more actively in some tubules. At about 15 to 20 days post partum in some tubules the first maturation division takes place in the primary spermatocytes, and the secondary spermatocytes are visible. The secondary spermatocytes are approximately one-half of the primaries in size. The nucleus of the resting stage contains just one or two small nucleoli. The second maturation division follows soon after the first one, and spermatids are found in the tubules by 20 days post partum. Spermatids are distinguished from the secondary spermatocytes by their smaller size, and their position situated nearer the center of lumen of the tubule. The elongation of spermatids is continued until the metamorphosis into spermatozoa are finished. In 25 days post partum the metamorphosis is taking place in many cells. At the age of 30 to 35 days immature spermatozoa with tails are attached to Sertoli cells by their heads. Mature spermatozoa being free in the lumen of the tubules are first found at the age of 40 to 45 days. At 45 and 50 days old testes contain large numbers of mature sperms. The first cleavage occurs about 24 hours after copulation. The ovum reaches two-cell stage about 24 to 38 hours, four-cell stage about 38 to 48 hours, eight-cell stage about 48 to 64 hours, morula stage about 68 to 80 hours and blastocyst stage about 72 to 82 hours. We can find ova being from one-cell to eight-cell stages in the oviduct, and more advanced stages in the uterus. The blastocyst is oriented so that the abembryonic side faces the point of first contact antimesometrially. After that the blastocoele expands and the inner cell mass grows. A single layer adjoining the blastocoele and surrounding the embryonic ectoderm is the entoderm. Thus blastocyst becomes to be the egg cylinder stage. The ectoderm of egg cylinder is divided into two parts. One of them gives rise to extraembryonic ectoderm; another to the embryonic ectoderm. About 5 days after copulation the proamniotic cavity appears in the embryonic ectoderm. By active growth of dorsal end of extraembryonic ectoderm the ectoplacental cone develops. At 6 days after copulation the first mesoderm cells are budded off from a narrow strip of embryonic ectoderm.
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