HYBRID NUTRITIONAL REQUIREMENTS 121 



others with C-4 but not with C-8, and still others develop none with either 

 C-4 or C-8. 



Dodge has suggested that the heterocaryotic hybrid may synthesize a full 

 quantity of growth substances or essential metabolites. Whereas the growth 

 of each of the parents is limited by their inability to synthesize adequate 

 quantities of one or more essential metabolites. 



Dwarf 16, for example, may be able to make adequate quantities of essen- 

 tial metabolites 1, 2, 3, and 4, but unable to construct enough of 5, 6, 7, 

 and 8. On the other hand, race C-4 may be unable to synthesize enough of 

 1, 2, 3, and 4, but be capable of producing an adequate supply of 5, 6, 7, 

 and 8. When nuclei of the two races are brought together in a common 

 cytoplasm, the essential metabolites synthesized by one of the nuclear com- 

 ponents supplement those synthesized by the other component. The hetero- 

 caryotic mycelium is then supplied with adequate quantities of all the 

 essential metabolites necessary for rapid growth. 



We have tried to test this hypothesis by supplementing with various 

 substances the medium on which race 16 and other dwarf races were grown. 

 If it were possible to increase materially the growth rate of the dwarf race by 

 supplements in the medium, without introducing the heterocaryotic condi- 

 tion, the limiting factors for dwarfness could be identified and the stimulus 

 involved in the heterocaryotic condition identified. 



A basal agar medium containing mineral salts, dextrose, asparagine, neo- 

 peptone, and thiamine was supplemented by a mixture of purine and pyrim- 

 idine bases; by a vitamin mixture containing PAB, calcium pantothenate, 

 inositol, nicotinic acid, pyridoxine, riboflavin, thiamine, guanine, hypoxan- 

 thine, and 2-methyl-l, 4-naphthohydroquinone diacetate; by malt extract, 

 casein hydrolysate, cow's milk, dried yeast, choline, a-tocopherol, hemin, 

 oleic acid, ascorbic acid (filtered sterile), coconut milk, Taka-diastase 

 (filtered sterile), water extracts of the mycelium of Neurospora, liver ex- 

 tracts (both filtered sterile and heated), adrenal cortical extract (unheated), 

 estrogenic substance, progesterone, anterior pituitary extract, posterior 

 pituitary extract, whey, or potato extract. 



None of the substances or combinations of them as used increased the 

 growth rates of any of the dwarf races to an extent adequate to explain 

 heterocaryotic vigor. Some beneficial effects, usually noted only in older cul- 

 tures, were obtained from cow's milk and from liver extract. These efifects 

 were not sufficiently marked to suggest that either supplement supplied the 

 missing factors. 



We were unsuccessful, therefore, in defining the factors limiting the 

 growth of the dwarf races and conversely those effective in inducing more 

 rapid growth in the heterocaryotic mycelium. 



Our failure may be explained in various ways. We may not have included 

 in our various supplements the missing essential metabolites. These metabo- 



