294 



PRINCIPLES OF EMBRYOLOGY 



of birth there is a great decrease in the length of leg relative to the re- 

 mainder of the body, whereas after birth the most important changes are 

 a lengthening and deepening of the body. In wild sheep the changes 

 are somewhat similar, though perhaps not so marked. In wild pigs there are 

 no very marked changes in proportions from the foetus up to the adult. 

 It is these basic developmental patterns which are altered either by the 

 genes selected by the livestock breeder or by the conditions of feeding 



Figure 13.6 

 Changes of proportion in the growing horse. The upper row of drawings 

 show a primitive type of horse at various stages from the foetus to the adult, 

 adjusted in size so as to have the same length of cranium. By the stage of the 

 first drawing (late foetus) there has already been a great development of the 

 legs; later there is a preponderant growth of the trunk. The two drawings 

 below show on the left a thoroughbred, and on the right a draught-horse ; 

 the later phase of growth has been minimised in the former and enhanced 

 in the latter. (After Hammond I950-) 



and husbandry under which he keeps his animals. Among horses, for 

 instances, the strains bred for speed have been produced by selecting 

 genes which partially suppress the later changes, so that the adult horse 

 retains the long legs and slim body of the normal juvenile phases. In the 

 heavy draught horses, on the other hand, genes have been selected which 

 increase these later changes so that one obtains an animal with a very 

 large and heavy body and relatively shorter legs. In the mutton breeds of 



