GROWTH IN SIZE AXD BODY PROPORTIONS IN FARM ANIMALS 333 



as a whole. Growth of the mammary gland during pregnancy is stimu- 

 lated by hormones coming from the placenta and the anterior pituitary. 

 These stimulate the alveolar cells and increase their metabolic rate, 

 thus giving them some priority for nutrients from the bloodstream. In 

 a young heifer, in which the other tissues of the body are actively 

 growing, there is strong competition from the other tissues, and so the 

 heifer's maximum daily yield of milk is below that of an adult cow 

 (Figure 12). These differences can be much reduced by high-plane 

 feeding during the last six weeks of pregnancy. In a young animal, 

 however, the aging of the gland is slow, and so its lactation cur\'e is 

 prolonged compared with that of the old cow, in which the growth 

 of the alveolar cells is at a maximum but aging of the cells is rapid 

 (Sanders, 1928). 



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