it is apparent that all modes of survival during starvation are largely dependent on the possession of energy reserves, and it is not therefore surprising that energy rich substances are quantitatively so predominant. It should be pointed out, however, that nitrogenous compounds may be stored as frequently as energy rich reserves, and their apparent scarcity may be simply a reflection of the relatively small amounts required by the cell; for a given amount of growth or metabolic transformation much more carbon and energy source is required, weight for weight, than nitrogen source, and the carbon and energy source is therefore always more obvious and more easily detectable. The purely technical difficulties of detecting a nitro- genous reserve may also play a part; the only known nitro- genous substances which might act as reserves are protein and nucleic acids, and present analytical techniques would make it difficult to detect small storage amounts of these compounds among the essential structural and functional proteins and nucleic acids of the cell.
In this connection there is the added difficulty that there is probably no hard and fast dividing line between materials that function as "expendable" nutrient reserves, and components of the organism that are "essential" to its existence. Under the stress of prolonged starvation, it is conceivable that an organism might, to a limited extent, utilise some of its "essential" constituents in order to maintain its viability. A rather specialised example of this phenomenon was demonstrated by Spiegelman and Dunn (194e). These authors found that when yeasts were adapted to the fermentation of galactose in the absence of an exter- nal nitrogen source, there was some loss of glucozymase activity. This loss could be prevented by the addition to the medium of a nitrogen source, and it was deduced that some of the glucozymase protein was being broken down and utilised for the synthesis of the galactozymase. Since failure to adapt would have resulted in starvation, one can regard this phenomenon as a specialised instance of a response to adverse nutritional conditions, a response which entailed the partial breakdown of an "essential" or "functional" (as opposed to reserve) cell constituent. These findings suggest the possibility that other components of the cell, normally regarded as "essential ", might be utilised similarly.