101 Rhode Island Red × Light Sussex chickens have been made to conform to four differently shaped growth-curves from hatching to 24 weeks, by control of their plane of nutrition. The four treatments HH, LH, HL and LL allowed comparisons to be made between birds of the same age but different weights.

As the main concern has been to find the essential nature rather than the precise extent of the effects produced by different planes of nutrition, the treatment differences were made as extreme as possible without allowing the rations to become unbalanced.

The conclusions reached are as follows:

1. The treatments used have produced marked effects on the growth-curves of the birds, analysed on the basis of equal age. These effects have been shown to be statistically significant at all stages after 3 weeks.

2. In body-weight and form, two types of bird were produced. The HH and LH birds were similar, with greater relative proportions of the late developing parts and secondary sexual characteristics, and the small differences that existed between them were not significant at 24 weeks. Likewise, the LL and HL birds formed a comparable group, exhibiting a more infantile body form similar to that of the HH and LH birds at an earlier age. The difference

between the HL and LL birds, though generally not statistically significant, is greater than that between the LH and HH group.

3. The treatments produced a small but consistent effect on the external measurements of the birds when comparisons were made on the basis of equal weight. Nine measurements were recorded. In the case of four measurements, no treatment effect was noted. Four skeletal measurements demonstrated a consistent effect of treatment, the LL and HL measurements exceeding those of the HH and LH at equal live-weights. This result was reversed in the case of the only muscular measurement, thickness of leg musculature. Opposite results are therefore indicated for the muscle and skeletal tissues. This finding will receive further discussion when the dissection analysis results are discussed in a later paper.

4. The presence of definite growth-gradients from the body extremities towards the trunk is indicated by the external measurements.

5. The two sexes differ in their live-weights at equal age and in their bodily proportions both at similar ages and similar weights. The most marked sex-difference in external measurement is noted in the length of the mid-wing, and a possible functional explanation is advanced. In general, the sex-difference is greatest for the late maturing characters.