1. Two series of half-brother-sister lambs of balanced sexes have been reared on widely different (High and Low) planes of nutrition from the end of the third month of foetal life to 41 weeks of age. The effect of this on the growth in live weight and relative development of the body proportions has been studied.

2. The feeding of the mothers of the experimental lambs during the last 2 months of pregnancy was such that the High-Plane mothers gained 39·5 lb. live weight during pregnancy as compared with a gain of 1 lb. only in the Low-Plane group. The single lambs from both groups were of equal weight at birth, while on the average each individual twin was 47% heavier in the High-Plane than in the Low-Plane group.

3. In post-natal life the High-Plane lambs were fed so as to allow them to grow at their maximum rate and the feeding of the Low-Plane ones was controlled so as to allow them to grow at a slow but uniform rate. Lambs from both groups were killed at birth, at 9 weeks old and at 41 weeks old. At 9 weeks old the Low-Plane lambs killed had an average live weight of 14·3 lb. and the High-Plane ones 61·5 lb.; at 41 weeks old the respective weights were 69·5 and 180 lb.

4. The study of the age changes in the High-Plane lambs further proved the theory put forward by Hammond (1932) that in the sheep in post-natal life the different anatomical regions and tissues of the body grow differentially and in a definite order of development. The dressed carcass is later maturing than the organs and offal parts as a whole.

5. Of all the organs in the body the brain is the earliest developing, followed closely by the eyes. The thoracic organs as a whole are earlier maturing than the abdominal organs. Of the former, the thyroids, lungs and trachea and the heart are the earliest maturing in that order, while the thymus glands are later maturing. Of the alimentary tract, the oesophagus and the abomasum are the earliest developing parts, while the rumen and reticulum are much later maturing, growing at nearly twice the rate of muscle in the carcass from birth to 41 weeks. Of the other abdominal organs, the kidneys are earliest maturing, growing at approximately similar rate as the lungs and trachea, and slower than the heart in post-natal life. The abdominal fats are the latest maturing of all the organs, and of these kidney fat is the earliest maturing and the caul fat the latest.

6. The marked heterogonic growth of the organs appears to be related to their function. Those organs of most vital function to the life of the animal like the brain, eyes, lungs, kidneys, heart, oesophagus, abomasum and small intestines, are relatively well developed at birth, and consequently grow proportionately less in post-natal life than organs like the rumen and reticulum, which have an unimportant function until after the lamb begins to consume fibrous foods, or those whose function is largely that of storage of nutrients, which develop mainly in the later stages of growth.

7. The development of the different joints or body regions exhibits a marked gradient of increasing growth rate from the head and feet to the loin region, the feet and head growing least and the loin most in post-natal life, while the legs, neck and shoulders are in an intermediate position, the legs being earlier maturing than the shoulders.

8. The major tissues of the body exhibit marked differential growth rates in post-natal life. The order of increasing growth rate with age follows an outward trend from the central nervous system to bone, tendon, muscle, intermuscular fat and subcutaneous fat. Consequently, the early maturing nervous tissue and the skeleton make a greater proportion of their growth earlier in life than does muscle and fat; the latter, particularly subcutaneous fat, does not develop to any great extent until late in the growing period.

9. Within any of the major tissues, bone, muscle and fat, well-defined growth gradients are observed. Within the skeleton, in post-natal life, waves of increasing growth intensity pass from centres of early (even foetal) maximum rate of growth near the extremities, towards the central region of the body. As regards the axial skeleton, the skull, or rather the cranium, is the earliest part to develop, and from it waves of increasing growth intensity pass backwards to the lumbar region and downwards to the nose and lower jaw. In each limb, similar growth waves pass with age from the early developing metacarpals and metatarsals (cannon bones) down to the distal bones and up towards the lumbar region of the body, the pelvis and scapula being later developing than the femur and the humerus. The ribs appear to be the latest developing bones of the body, while the sternum situated further down in the body, though late maturing, is, however, earlier maturing than the scapula and the ribs. The bones of the fore-limb appear somewhat later maturing than those of the hind-limb.

10. As regards growth in length and thickness of the long bones of the limbs, a similar wave of increasing growth intensity passes from the cannons up the limbs. The upper bones increase relatively more both in length and thickness than the cannons after birth, and growth rate in length is at its maximum at an earlier age than growth rate in thickness.

11. In the case of muscle and fat, gradients have been demonstrated in their order of development from the head and neck backwards, and from the lower parts of the limbs (arms and legs) upwards to the loin region.

12. Changes in carcass measurements are of the same order as the changes in weight and/or shape of the tissues and parts measured, thus confirming their values as indices of carcass composition as well as conformation.

13. Marked differences between the sexes are apparent. The High-Plane males were on the average 15% heavier at birth than the females, which was found to be significant at the 5% level. At 41 weeks the wethers weighed 215·5 lb. and the ewes only 144·75 lb. This great difference appears to be principally caused by a longer span of growth in the wethers and by their higher rate of growth during the latter part of the growing period, rather than being due to their higher birth weight, for the females grow at a faster rate immediately after birth, and at 2 weeks of age have reached a greater average live weight than the males.

14. At 9 weeks of age the ewes were in a more advanced stage of development in respect of all characteristics than the wethers. The latter were proportionally less developed in all the late maturing characters such as dressed carcass percentage, had less fat and more bone, and each tissue was proportionately less developed in the late maturing joints in the wethers than in the ewes.

15. At 41 weeks the picture was completely reversed in every respect. The wethers at that age had not only reached 50% greater weight than the ewes but all their tissues and body proportions were in a more advanced stage of development, i.e. the later maturing tissues and anatomical regions were proportionately better developed in the wethers than in the ewes. At both ages these differences were reflected in photographs of the carcasses and the limb bones as well as in the carcass measurements.

16. The vastly different quantitative planes of nutrition of the High- and the Low-Plane lambs did not affect all the parts and tissues of the body equally, so that the Low-Plane lambs were not miniature images of the High-Plane lambs at the same age. On the contrary, the restricted nutrition of the Low- Plane lambs affected the different anatomical regions, organs and tissues of the body differentially and in an orderly manner. With but few minor exceptions, the different organs, tissues and anatomical regions of the body have been retarded in development by the restricted nutrition, in the direct order of their growth intensity. At any stage, an organ, part or tissue of high natural growth intensity at that stage was proportionately more retarded in development than organs, parts or tissues of lower growth intensity at that age. Over the whole experimental period the later developing parts or tissues were proportionately more affected than the earlier developing ones, which can only be explained in the same way as McMeekan (1940,1941) did in the pig, that the earlier maturing parts or tissues have a priority claim for the limited nutrients available in the blood stream when the growing animal is insufficiently fed to provide all parts or tissues of the body with an adequate nutritive supply for normal (maximum) growth. This holds at any stage of development, but at birth the differential effects of the restricted nutrition were much less pronounced than at 9 and 41 weeks.

17. The dressed carcass is more affected by the restricted nutrition in post-natal life than the organs and offal parts with the exception of the abdominal fats.

18. Of all the organs, the earliest developing brain and eyes are least affected in the Low-Plane lambs. At 41 weeks the brain is only 5% heavier in the Highthan the Low-Plane lambs, which is statistically insignificant for the available degrees of freedom; as the difference was proportionately greater at birth, poor nutrition in post-natal life appears to have no effect on the brain weight, while it may do so to some extent in foetal life. The ewes have a significantly lighter brain than the wethers (corresponding to their smaller live weight at birth). The eyes, though but slightly affected compared with other organs and body parts, are significantly lighter in the Low-Plane series at 41 weeks.

19. The restricted nutrition in foetal life did not have any effect on the weight of the lungs at birth and but little on the weight of the heart, while it greatly depressed the development of the thymus glands and that of the oesophagus, abomasum, spleen and somewhat less that of the liver, pancreas and kidneys. The development of the digestive tract and the abdominal organs was more retarded than that of the thoracic organs at birth in the Low-Plane lambs. In post-natal life the earlier maturing organs are in general less affected by the restricted nutrition than the later developing ones, the only important exception being the alimentary tract, which though growing at a relatively faster rate in post-natal life than the thoracic organs as a group, is less depressed by the low level of nutrition.

20. An organ of great growth intensity at any particular age interval is much more affected by the restricted nutrition during that interval than at other ages, i.e. the abomasum before birth, the rumen and the thymus glands from birth to 9 weeks, and the caul and kidney fat from 9 to 41 weeks old.

21. At birth the restricted nutrition in foetal life has had but small effect on proportional development of the different body regions. The head, however, is least and the legs most affected. In post-natal life the various joints are affected by the poor nutrition to a very different extent, and at each age interval they are, with minor exceptions, affected in the direct order of their growth intensity. At 41 weeks the head is 1·6 and the feet 1·8 times as compared with the thorax 4 and loin 4·4 times heavier in the High- than the Low-Plane lambs.

22. At birth the major tissues of the carcass are differentially affected by the restricted nutrition, the brain being least affected followed by the skeleton, muscle, tendon and fat. In post-natal life these tissues are affected in the order of increasing growth intensity, with age; the brain is least affected, followed by the skeleton, muscle, intermuscular fat, kidney fat and subcutaneous fat.

23. Restricted nutrition produces also differential effects on the relative development of the different anatomical units within each of the three major tissues, bone, muscle and fat. As regards the skeleton, individual bones or skeletal parts are retarded in development in direct relation to their growth intensity at each age interval. At birth the earliest maturing bones, i.e. those with high growth rate in foetal life, are relatively less developed than the other bones in the Low-Plane lambs, while at later ages the development of the late maturing bones is proportionately much more affected.

24. Not only does the plane of nutrition affect the weight of the individual skeletal units differentially, but it also affects the form of the bones in a comparable way. The late developing growth in thickness is retarded by poor nutrition to a much greater extent than the earlier developing length growth in postnatal life, while in foetal life the length growth of the early maturing cannon is more affected than its late maturing growth in thickness, the latter having very low growth intensity in foetal life as compared with the length growth. The differential effect on the form of the metacarpals over the whole experimental period is so great that those of the Low-Plane lambs at 41 weeks resemble bones of semi-wild or unimproved late maturing breeds, being very thin and round, while the cannon bones of the High-Plane lambs at 9 weeks and much more so at 41 weeks, have the thick flattened shaft characteristic of early maturing improved breeds.

25. The effect of the Low Plane of nutrition on the development of muscle, intermuscular fat and subcutaneous fat, as in the skeleton, exhibits a gradient from the early to the late maturing regions of the body in each case, i.e. the latest maturing parts are most and the earliest developing least affected.

26. Marbling or intermuscular fat as measured by chemical analysis of a sample of the longissimus dorsi muscle is affected by the Low Plane of nutrition to a greater extent than intermuscular fat, but less than subcutaneous fat. The plane of nutrition also affects the water content of the muscle, it being 3% higher at 9 weeks and 2% higher at 41 weeks in the Lowthan the High-Plane lambs.

27. The resultant effects of the plane of nutrition upon the body conformation and composition in terms of its major tissues is clearly reflected by the various carcass measurements taken.

28. The wethers appear to be more sensitive to restricted nutrition than the ewes and more so in the late maturing tissues and anatomical regions of the body. The proportionately more advanced development of the High-Plane wethers at 41 weeks as compared with the ewes is more than completely suppressed by the restricted nutrition of the Low-Plane wethers, the latter being in a less advanced stage of development than the ewes of the same treatment group at the same age.

29. Though an animal which has been retarded in its development by restricted nutrition retains more of the juvenile form than another of the same age which has been growing to its full inherent capacity, it is, however, not merely anatomically younger, corresponding to the weight difference, because of the differential effects of the restricted nutrition on the different organs, parts and tissues of the body. That even within a tissue some parts are penalized proportionately more than others by restricted nutrition during a period of active growth, has been tested statistically in the skeleton and found to be true.

30. Practical applications of some of the results are discussed.