Nutrition I35 



relations within the Porifera. The principal sterol of insects and vertebrates is 

 cholesterol. 



Unknown Factors. The discovery of vitamins often proceeds by a tedious 

 process of dietary elimination; an animal is fed a complex diet on which it 

 thrives, then this diet is gradually fractionated into identifiable components 

 which can be eliminated one by one. Sometimes the searches for an essential 

 component for two or more organisms lead to discovery of the same substance, 

 as when folic acid was identified as a factor for certain yeasts, molds, and 

 mammals. A few examples of dietary factors which have not been identified 

 chemically can be cited. 



The ciliate, Tetrahymena geleii, has been cultured in a medium of amino 

 acids, dextrose or acetate, inorganic salts, and B vitamins as described above. 

 In addition, these organisms require a purine and a pyrimidine and a third 

 factor which is obtained best from a Norit filtrate of liver extract. This un- 

 known liver factor has been separated into tv/o components (factors II A and 

 II B). "'• '*=■ i-^-^ In turn factor II B is found to be replaceable by large amounts 

 of pyridoxine together with copper and iron salts. Factor II A is not replace- 

 able by any of a variety of compounds, including vitamin Bi-; it is stable to 

 hydrolysis in weak acid and alkali, and is precipitated with lead and extracted 

 with butanol. Its identity is not known, but it has been named protogen. 



Trout which are fed on a diet of devitaminized casein, sucrose, salts, known 

 vitamins, and dry liver, which is adequate for rats and chickens, do not grow 

 well. In addition to thiamine, riboflavin, pyridoxine, pantothenic acid, and 

 choline, trout require some unknown substance which is present in fresh liver 

 and yeast. '•^" It will be of interest to test vitamin B12 on trout. 



Planaria grow rapidly in a sterile medium if fed on animal tissues which 

 provide a complete supply of needed factors. Liver is the most favorable food; 

 the mucosa of such regions as cecum is better than that of other regions such 

 as duodenum; egg white alone is deleterious, whereas egg yolk permits slow 

 growth; and kidney cortex is superior to kidney medulla. ^^^ ^^- The liver 

 from guinea pigs fed on a balanced diet containing grass or kale permits good 

 growth of planaria, but if the green vegetable is replaced by orange or tomato 

 juice, the planaria cease normal growth. ^■'- Guinea pigs on such a deficient 

 diet ultimately show various deficiency symptoms such as muscle degenera- 

 tion. ^"'•■^ This guinea pig liver factor for planaria is somewhat similar to the 

 unknown factor occurring in liver which is required by trout. There are 

 several reports of unknown factors which can be supplied from liver as essen- 

 tials for normal growth and maintenance of guinea pigs. ^'''^ 



Knowledge of the physiology of parasitic worms has long been retarded by 

 inability to culture them in vitro. Many attempts at culture have been made, 

 but the best that can be done at present is to prolong survival. Some tape- 

 worms and nematodes can be kept for days in salt solutions. The most success- 

 ful cultures to date are those of Smyth, '-**- '-" who kept larval tapeworms in 

 a medium of peptone broth, salts, and sugar for 300 days, during which there 

 was some maturation of the worms and regular utilization of sugar. Whether 

 parasitic worms can absorb essential factors from host tissues is not well estab- 

 lished. However, tapeworms grow well in rats which are receiving a diet low 

 in protein or in vitamins A, D, E, and B], but not in rats fed a diet deficient 

 in carbohydrate or riboflavin (in female but not in male hosts); hence these 



