40 PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 



Group II at different ages, from the data iu Table 13. The results of 

 this calculation are given in Table 21. 



Although at the younger ages the composition of the added tissues in 

 the Group I steers over the Group II steers probably indicates a greater 

 attained growth, at 36 months of age and after, the results are not incom- 

 patible with the belief that much the greater part of this added tissue is 

 adipose tissue. 



It may be concluded, therefore, that the steers of Group I probably 

 grew at a somewhat faster rate than the steers of Group II for the first 

 two or two and a half years, but that their greater content of nitrogen 

 at all ages and their greater rate of nitrogen retention for the first two 

 years was to a considerable extent accounted for by the greater pro- 

 toplasmic content of their adipose tissues accompanying fattening. 



TABLE 21 



The Percentage Composition of the Average Difference in 



Empty Weight between the Steers of Group I and 



Those of Group II at Different Ages 



The histogenesis of adipose tissue. — The histological work of Bell(*^), 

 carried out in close connection with the Missouri experiments on growing 

 and fattening steers, indicates clearly that the formation of adipose tissue 

 involves both cellular proliferation and cellular enlargement. In the 

 formation of intramuscular as well as subcutaneous adipose tissue, fat 

 cells are formed around the blood vessels, the process extending out on 

 the smaller vessels as the animal fattens. Apparently the fat passes out 

 of the blood stream and is taken up by the adjacent cells. Acting directly 

 upon the relatively undifferentiated connective tissue cells, the fat (in 

 some soluble form) causes them to pass into the preadipose and later 

 into the adipose condition. Thus, according to Bell, " the blood vessel is 

 the center around which the fat lobule develops. Whether in a mass of 

 preadipose tissue, or in ordinary connective tissue, the first fat cells 

 appear immediately around the blood vessels. The lobules thus estab- 

 lished increase in size to a large extent by the addition of cells adjacent 

 to the periphery. The increase in the number of fat cells is, however, to 

 a considerable extent due to the division of fat-free cells inside the 

 lobule.'' The occurrence of cellular proliferation was directly demon- 

 strated by the finding of mitotic figures and of old and young cells. 



