88 COMPARATIVE PHYSIOLOGY 



nitrogen supply, the sterile larvae were able to grow and pupate 

 normally. It thus appears that yeasts are the nitrogenous 

 food of Drosophila ; the simplest nutrient solution suitable 

 for the yeasts (and certain other micro-organisms) will replace 

 fermenting fruit in the ecology of Drosophila larvae. These 

 experiments of Baumberger are extremely suggestive in relation 

 to the diet of wood-boring animals, the significance of fungus 

 gardens, the curious habitat of such organisms as the vinegar 

 worm, and a host of other bionomic problems. 



Researches have also been carried out on Drosophila 

 larvae in relation to the accessory food-factors or vitamins. 

 The term " vitamin " is one which at present can hardly be said 

 to convey more than a recognition of our failure to induce 

 mammals to grow healthily on a diet of purified carbohydrate, 

 fat, protein, etc., and our almost complete ignorance of those 

 constituents of natural foods which must be added to such a 

 diet to preserve health and normal development. The necessity 

 for recognising accessory food-factors was first clearly recognised 

 by Hopkins (1906), and the conception became more concrete 

 when Funk (191 1) extracted from 100 kilograms of yeast 

 2*5 grms. of a material of which a dose of 2 mg. sufficed to cure 

 the polyneuritis induced in pigeons by an exclusive diet of 

 polished rice. There are at least three chemical entities 

 included under the term '' vitamin." A is present especially 

 in animal fats, B in yeast, and C in fruit juices ; but in fresh 

 animal or vegetable food all three are represented to some 

 extent. The separate identity of these substances is inferred 

 from the different clinical results of eliminating one or the 

 other. 



Bacot and Harden (1922) have investigated the extent 

 to which vitamins are essential to the diet of Drosophila. 

 Successful growth of larvae can apparently proceed in a nutrient 

 medium composed of pure caseinogen, starch, sugar, and salts 

 only if small quantities of yeast extract (as a source of " B ") 

 and traces of butter fat ('* A ") are included. '' C " was not 

 found to be essential, though amply available in the normal 

 diet (fermenting fruit-juice). 



The behaviour of Drosophila in regard to yeasts more- 



