RESUMO
With the Deeley integrating microdensitometer incorporating a crushing condenser, the Feulgen stain content of 19 species of placental mammals, birds and reptiles was determined. Arranging the species as a series of pairs, comparative DNA values were obtained which are by and large in good agreement with previous results obtained from two-dimensional measurements of chromosome area. 1. Placental mammals as a whole constitute one uniform group with regard to total genetic content. There was no significant difference in DNA values between man, the horse, the dog, the golden hamster and the mouse. The creeping vole with the extremely low diploid chromosome number had a DNA value 10 per cent lower, regarded as a reflection of the loss of centromeric heterochromatin which accompanied a drastic decrease in diploid chromosome number. 2. Four representatives of the class Aves also constitute one uniform group with the DNA value 44 to 59 per cent of that of placental mammals. 3. The class Reptilia falls into two fairly discrete groups. Six representatives of the order Squamata have a DNA value 60 to 67 per cent of that for placental mammals, while three species representing the order Crocodylia and Chelonia have 80 to 89 per cent.