GROWTH IN SIZE AND BODY PROPORTIONS IN FARM ANIMALS 



325 



with age (Hunter, 1956), although weight differences persist for many 

 months ( Figure 5 ) . 



In the fetal stage, nutrition is derived by transfusion from the 

 maternal bloodstream and so is affected not only by the placental area 

 but also by the level of nutrition of the mother, as we have seen ( Fig- 

 ure 2 ) . When the area of the placenta is small, as in twins or triplets in 

 sheep, tlie level of the mother's nutrition is more important than it is 

 when the area is large, as in singles. At this stage there is very rapid 

 growth in the fetus itself and cessation of the growth of the placenta. In 

 muscle, which by weight is the major body tissue, increase in size, 

 which has hitherto been by increase in cell numbers (Figure 6), now 

 takes place by increase in cell size ( Joubert, 1955 ) . In this tissue all the 

 further increase in weight up to adult life is by increase in muscle-cell 

 size. This means that by the beginning of the fetal stage, the maximum 

 adult size of the animal has been fixed, since differences in size be- 

 tween breeds within a species are due to differences in muscle-cell 

 numbers and not to cell size. 



This also applies to sex differences in size within a breed. For 

 example, rams and wethers are larger than ewes, so that in young sheep 



Figure 4. The effect of maternal nutrition on the size of the lamb at birth. 

 At the right is a purebred Welsh lamb with its mother. At the left is a pure- 

 bred Welsh lamb of the same age which was transplanted as a fertilized egg 

 into a Border Leicester ewe, standing beside it. Despite the same heredity, 

 the lamb incubated by the larger mother is bigger than its normally bred 

 fellow. (Hunter, 1956.) 



