Of 158 ICR-JCL mice (16 litters, _??_74, _??_ 84), the organs (brain, heart, liver, spleen, kidney, testis and ovary) were weighed from the 1 st to the 10th week (3 to 71 days) after birth. Each organ was isolated from the body after killing by ether anesthesia, and was weighed by a torsionbalance.
The results obtained were as follows:
1) Maturation period of the brain was 5-6 weeks after birth. It was one to two weeks earlier than those of the other organs, showing the neural type growth.
2) Growth curves of the heart, liver and kidney were similar to that of the whole body. The curve of the spleen showed an exceptional pattern, in which the peak appeared at 3-4 weeks after birth.
3) Genital organ weights increased continuosly until the 10th week, and they did not attain a plateau during this observation period.
4) Two inflexion points appeared clearly, at 3-4 weeks and 7-8 weeks after birth in growth curves of all investigated organs. This characteristics was in accordance with the results which were observed in external morphological measurings.

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To examine the serological and histological reactions of mice toEscherichia coli, germfree mice at the age of 4 weeks were given perorally a strain of E. coli.In the feces and gastrointestinal contents of these monocontaminated animals of the first generation of gnotobiotic life (gnotobiotic mice I) and their offsprings (gnotobiotic mice II), a large number of organisms were detected, as shown in Fig. 1. The organisms were also recovered from the cranial mesenteric lymph nodes, the liver and the lungs of the two groups of gnotobiotic mice.
Serum antibody titers to theE. coliorganisms examined by indirect hemagglutination methods were shown in Table 1. An elevation in agglutinin titers toE. coliwas observed with all of the gnotobiotic mice I, while the antibody was detectable at a low titer in only a half of gnotobiotic mice II. No detectable antibodies to theE. coliorganisms were present in the majority of our germfree and conventional mice. Some immature secondary nodules and some plasma cells in the medullary cords existed in the cortex of lymph nodes of the gnotobiotic mice I and II, while few of them were detected in germfree mice.

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The incorporation of ¹⁴C-Fructose was studied in the mice exhausted by swimming and fast by means of whole body autoradiography.
Animals tired with swimming for consecutive 9 days lost their weight markedly and the incorporation of ¹⁴C clearly increased in the liver, brain and superficial muscles of the hind limbs as compared to control animals.
In the case of animals fasted 3 times for 24 hours every three days, average body weight did not decrease remarkably and the organ radioactivities were comparable to those of control except for the liver.
In the extremely exhausted animals after swimming plus fast, the uptake of ¹⁴C increased remarkably in most tissues.

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The causative organism of the mouse megaenteron (Escherichia coli0115 a, c : K (B) ) was examined in 16 mouse breeding colonies which distributed in Kanto, Tokai and Kyushu districts of Japan. A total of 1, 242 mice aged 3 to 4 weeks were collected from the colonies and sacrificed for examination on the megaenteron and for isolation of the organism from the large intestine. The organism was detected in 11 of the 16 colonies. The isolation rates in these colonies were 33.7%, 16.2%, 11.5%, 7.6%, 6.0%, 5.8%, 3.7%, 3.7%, 1.9%, 1.8% and 1.8%. The lesion of megaenteron was observed in 41 of 231 mice obtained from a colony which showed the highest isolation rate of the organism.
To examine the pathogenicity, every 59 of 138 isolates were orally inoculated into DDY suckling mice aged 1 week with 10⁴ bacterial cells. Thirty-six strains isolated from 3 colonies (A, B, F) were highly pathogenic to the sucklings, producing the intestinal lesion of the megaenteron accompanied with a severe diarrhea. Although many of 18 strains originated from 7 colonies (C, D, E, G, H, I, P) had the ability to produce a severe diarrhea, occurrence of the megaenteron limitted to 80 to 25 per cent of the infected sucklings. Five strains from the remaining one colony (J) produced either or both a diarrhea and the megaenteron only in a few of sucklings used, in spite of the fact that they developed themselves in the intestines of all the mice.

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