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1. The brain of Octopus vulgaris grows allometrically with respect to weight from 12 mg. at the beginning of post-larval life to 1.95 g. at a body weight of 8.5 kg. 2. The two optic lobes contribute 64% to the wet weight total at the beginning of the growth range and 44% at the end. 3. The 150-fo1d increase in brain weight is accompanied by a 40-fold increase in total DNA content, 80-85% of which is in the optic lobes. DNA concentrations of the order of 20 mg./g. wet weight are found in the vertical lobes. 4. An average of 1 mg. DNA is yielded by 1.9x108Octopus sperms and this figure is used to calibrate the nucleic acid content of the optic lobes, and of the suboesophageal and supraoesophageal divisions of the brain in terms of their ‘diploid equivalent’. 5. The ratio between the ‘diploid equivalent’ and the numbers of neurones counted histologically is highest for the suboesophageal division and lowest for the vertical lobes and probably indicates a high glia/neurone index in brain areas with many large cells and low index in those with a homogeneous population of small cells. The brain cells of O. vulgaris do not appear to be polyploid. 6. The RNA/DNA ratio is less than 1.0 in the optic and supraoesophageal lobes, higher than 1.0 in the suboesophageal lobes. The ratio increases during growth, especially in the suboesophageal division.

Relative Growth, Nucleic Acid Content and Cell Numbers of the Brain in Octopus Vulgaris (Lamarck)