An unknown organism that can utilise NH ₄ Cl as nitrogen source was grown for several generations under four different conditions as given below: (i) medium containing ¹⁵ NH ₄ Cl (ii) medium containing ¹⁴ NH ₄ Cl (iii) medium containing ¹⁵ NH ₄ Cl followed by culturing in medium containing ¹⁴ NH ₄ Cl for one generation (iv) medium containing ¹⁵ NH ₄ Cl followed by culturing in medium containing ¹⁴ NH ₄ Cl for two gen