1. Evidence is summarized for the constancy of DNA content for each set of chromosomes in the various cells of an organism.

2. The DNA contents of the egg and sperm nuclei are the same.

3. A brief survey is given of DNA contents per cell in invertebrates and vertebrates.

(a) In invertebrates there is some slight evidence that when primitive and higher forms are compared the amount of DNA per cell is increased in the latter.

(b) In fishes there is a tendency for the amount of DNA per cell to remain constant within the different species of a family.

(c) The values of DNA per cell in lung fishes, amphibians, reptiles, and birds suggest that in the evolution of these vertebrates there has been a decline in DNA content per cell.

4. Concerning the significance of quantity of DNA per cell in vertebrates:

(a) It appears not to be in proportion to the number of different genes in a cell.

(b) It may be related to the number of strands in the chromosomes.

(c) In homologous cells of different animals it is directly related to the mass of the cell.

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