VITAMIN A 261 



and in sufficient numbers to avoid vitiation of results through indi- 

 vidual variability of the animals. Hence, animals making equal gains 

 in weight during the experimental period may be regarded as receiving 

 equal amounts of vitamin A, and the richness of different foods in 

 vitamin A may be taken as inversely proportional to the amounts of 

 the foods required to furnish that fixed amount of the vitamin which 

 will produce a given result in a standard experimental animal in or 

 for a given time, as, for example, a given rate of gain in weight during 

 the experimental period in a rat that has been prepared for the purpose 

 in the manner described above. 



It is plain, as Steenbock and Coward (1927) point out, that the 

 cure of ophthalmia is more specific than response in growth; but it 

 has not been demonstrated that this criterion lends itself so well as 

 growth to accurate quantitative measurement of the vitamin. 



Weekly records of weights, food consumption and careful notes 

 on the condition of the animals should be kept. Only clean sterilized 

 apparatus should be used. Laboratory conditions should be controlled 

 so that dust, fumes, noise and fluctuations in temperature are reduced 

 to a minimum. At death or at the end of the experimental period all 

 animals should be autopsied. This procedure serves to detect any pos- 

 sible cases of abnormality due to causes unrelated to the vitamin de- 

 ficiency; and to confirm the conclusions drawn from the weight curves 

 as to the extent to which the vitamin deficiency, whether absolute or 

 relative, has affected the life and growth of the experimental animal. 



It is suggested that each animal be autopsied with all these points 

 in mind. If done thoroughly, the autopsies will acquaint the worker 

 with the characteristic effects of the vitamin A deficiency and will help 

 to increase our knowledge in this direction. 



In the experience of the Chandler Laboratories of Columbia Uni- 

 versity, about 85 per cent of the rats placed, when 4 weeks old, upon 

 the vitamin-A-free diet have developed the ophthalmia first noted by 

 Osborne and Mendel (1913a) as characteristic of the results of this 

 dietary deficiency. Pus in one or more of the glands near the base of 

 the tongue was found in 76 per cent of the "negative controls" ex- 

 amined for this sign, which was, therefore, almost as characteristic 

 of the vitamin A deficiency as was the ophthalmia. These animals, 

 placed at an early age upon a diet entirely devoid of vitamin A and 

 dying usually within about two months thereafter, did not show the 

 frequency of lung trouble stated by Steenbock and Nelson (1923) to 

 be one of the characteristic effects of vitamin A deficiency. There is, 

 however, probably no real discrepancy between the work of the two 



