In the present study, the authors investigated the skeletal muscle growth of the Barred Plymouth Rock chicken, and sought the normal age changes and the sex differences of the proportions of the muscle and other several components of body. The following parts were weighed; live body, fat, skin, viscera, abdominal fat, bone, intermuscular fat and skeletal muscle. The resu1ts obtained were as follows: 1. Sex differences of the live weight was a little at the age of zero week, and not recognized at one week. It was led to an apparent increase, however, after two weeks of age, particularly in the period from 10 to 20 weeks in advantage of the male. The live weight of the male was by about 700 g heavier than that of the female at the age of 31 weeks. 2. The skeletal muscle is a late-maturing organ, because the muscle weight expressed as the relative increase from zero to 31 weeks of age is about twice as large as that of the live weight. However, the skeletal muscle matured earlier than the abdominal fat in both sexes. In the female, it also matured earlier than the intermuscular fat and skin. 3. The muscle weight showed a rapid rate of increase from zero to two weeks of age, and it increased its initial weight by 4.7 times in the male and 4.6 times in the female for the two weeks. The growth rate of the muscle per week was the largest at the age of about 10 weeks, and larger until about 20 weeks of age. 4. Sex difference in the gain of the musc1e weight was only a little until two weeks of age, but became conspicuous gradually from two to 18 weeks. It was apparent from 18 to 25 weeks of age, and the most conspicuous from 25 to 31 weeks. The difference of the muscle weight between both sexes was about 450 g at the age of 25 weeks and about 590 g at 31 weeks in the advantage of the male, respectively. 5. The muscle of the male showed a larger percentage of the live weight than that of the female after the age of 18 weeks. The difference in the percentage of the muscle weight to the live weight between both sexes was 8.0% at the age of 25 weeks and 11.2% at 31 weeks. The percentage of the muscle weight to the live weight showed a rapid increasing rate from the age of zero to two weeks in both sexes. 6. The viscera matured most early among these six body components examined; the bone came second; next in order were the skin, the intermuscular fat and the muscle in the male, while the muscle, the skin and the intermuscular fat in the female. The abdominal fat matured most late among these six components in both sexes.
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