IV. ESTIMATION 



219 



liver oil was the official standard of reference, but vitamin D3 gives re- 

 sponses not very different). At the end of the assay the reference group 



-Cod liver oil- 



Irradiated ergosterol 



wmmm 



m 



m 



I. U. of vitamin D per 100 gm. of diet — 

 Upper numbers for cod liver oil 

 Lower numbers for irradiated ergosterol 



mffl 



Fig. 17. Response of chicks to graded doses of cod liver oil and irradiated ergos- 

 terol assayed with rats, showing that efficacy ratios vary with the dose. (After Mas- 

 sengale and Bills. ^'') 



shows a bone ash of, say, 42.3 %. The assay group has been given an unknown 

 cod liver oil, the amount by weight being the same as that of the reference 

 oil. This group shows a l)one ash of, say, 44 %. On the pass-or-fail basis, and 

 without reference to the curve, it can be said that the unknown oil contains 



