GROWTH IN SIZE AND BODY PROPORTIONS IN FARM ANIMALS 



323 



9f days after service 



afttr service 



Figure 2. The effect of the level of nutrition of the ewe on the size of the fetus at dif- 

 ferent stages of pregnancy. Note that the level of nutrition has no effect up to 91 days of 

 pregnancy, but a marked difference in fetal size appears by the 144th day of pregnancy. 

 All animals were photographed to the same scale and only twin lambs were used. (Wal- 

 lace, 1948.) 



of the mother ( Wallace, 1948 ) . At this stage the placenta continues to 

 grow, and the extent of this growth determines the amount of nutrition 

 the young animal will receive at the following fetal stage ( see Figure 

 2), when the trophoblast degenerates and nutrition is maintained by 

 diffusion from the maternal blood stream. Examples of this are seen in 

 the differences in birth weight between lamb singles and twins, caused 

 by the fact that whereas in a single birth the average number of coty- 

 ledons in the placenta is 83, in twins the number in each placenta is 

 only 57, so that the weight of the latter is 110 grams, as compared witli 

 538 grams for a single placenta ( Wallace, 1948 ) . 



Again, the size of the placenta will vary with the size of the uterus, 

 so that the size of the mother will determine the size of the young at 



