GROWTH 295 



sheep it is again genes which encourage the later changes in body confor- 

 mation which are required and in the improvement of pigs it is also the 

 late-developing hindquarters rather than the early-developing head and 

 forequarters that it is necessary to emphasise. 



The changes in body proportions can be affected not only by genes 

 but also by the level of nutrition on which the animals are kept. For 

 instance, if two comparable sets of pigs are kept, one on a high plane of 

 nutrition and another on a low plane, until they both reach the same 

 weight, it will not only be found that the high-plane pigs reach the speci- 

 fied weight more quickly, but that the two sets of animals differ in con- 

 formation at the end of the experiment. The low-plane animals retain a 

 more juvenile shape. One can say that in spite of maturing more slowly 

 they mature in a conformation which is characteristic of a younger animal 

 than do the similar pigs reared on the high plane of nutrition. Hammond 

 explains the situation in the following ways: The different regions of 

 the body, such as the head and the forequarters, the hindquarters, etc., 

 attain their greatest rate of relative growth in succession. The same 

 is true of different tissues, such as bone, muscle, fat, etc. The funda- 

 mental sequence in which these various parts come to the fore is never 

 altered, but changes in genes or changes in nutrition can either compress 

 the sequence into a shorter length of time or spread it out over a longer 

 interval. High nutrition brings the successive phases nearer together in 

 time; so do the genes which determine the draught type of horses or the 

 meat-producing type of sheep or cow. Low nutrition spreads the phases 

 further apart and so do the genes for racehorses (Fig. 13.7). 



It appears from other experiments of Hammond and his associates that 

 even if the plane of nutrition restricts the outward expression of the 

 sequence of phases the basic physiological changes determining them may 

 be proceeding nevertheless. Thus two sets of pigs were brought to the 

 same weight in the same length of time, but by different routes ; one 

 being kept first on the high plane and later on a low, the other first on a 

 low and later on a high plane of nutrition. They nevertheless showed 

 characteristic differences in conformation. There is obviously a great deal 

 more work to be done on the physiology and the genetics of these rela- 

 tions. It is one of the most fascinating, and also most practically important, 

 aspects of the whole subject of growth. 



Our knowledge of the mechanisms which control growth patterns is 

 very meagre. We have to recognise in the first place that the processes 

 which finally issue in an adult shape may be very complex. Waddington 

 (1950^7) has discussed one particular case from this point of view, that of 

 the wings o£ Drosophila and shown how the fmal shape depends not only 



