128 



VITAMINS A AND CAROTENES 



within the normal hmits of variation, Init the layers of growth and matura- 

 tion are increased at the expense of the proliferative layer. Experiments 

 of longer duration with lesser dosage reduced the epiphyseal cartilage to 

 the state found at completion of growth. In the dog and the chick complete 

 studies have not been made, although the same acceleration of epiphyseal 

 cartilage sequences results. Concurrently with epiphyseal cartilage acceler- 







'.I. I lypci \ It ;iiuiii()sis A in the dog. Cross section of ilic IiImiIm Ikhh a pui)i)y 

 that had received 300 I.U. daily for 68 days from the age of 2 months. The arrow 

 points in the direction of movement of the fibula as growtli of the leg proceeds. The 

 photomicrograph shows evidences of accelerated compact hone formation in the re- 

 maining fragment of old bone, many osteoclasts on the side undergoing resorption 

 and much newly formed appositional bone on the opposite side. Cross sections of a 

 fibula from a normal litter mate show the same pattern of resorption on one side and 

 api)ositional bone growth on the side facing the direction of movement resulting from 

 growth. 



ation there is a great acceleration of the processes of resorption of bone 

 and new bone formation essential to the remodeling that accompanies 

 growth. This can be most easily followed at the ends of long bones where 

 normal remodeling results in reduction of cross-sectional diameters and 

 where it is achieved through resorption from without (periosteal surface) 

 and new formation from within (endosteal). Resorption is by osteoclasis, 

 and the histologic details are precisely those of normal osteoclasis. The 

 new bone formation also is normal in histologic detail, as seen in appositional 

 bone formation. The pattern exhibited in the accelerated remodeling cor- 

 responds precisely with that of normal growth, symmet rical or a.sj^mmetrical 



