Measured by a cell dispersion technique, the neuron population densities of the macular projection areas of 23 individuals aged 20 to 87 years fell from about 46 million neurons per gram of tissue of age 20 to about 24 million neurons per gram at age 80. A corresponding increase in glial cells was found in the gray and subjacent white matter. Since the older brains showed a significant decrease in mass, the decrease in neuron population density could only reflect an absolute loss of neurons proportional to or greater than the nearly 50% loss indicated by these data. Neuron death in the macular projection area of the human visual cortex is evidently a major factor in the loss of visual acuity in old age.

The worldwide pattern of movement of DDT residues appears to be from the land through the atmosphere into the oceans and into the oceanic abyss. Calculations based on the fragmentary data available on. rates of movement and sizes of various pools of DDT residues lead to the conclusion that concentrations in the atmosphere and in the mixed layer of the oceans lag by only a few years behind the amounts of DDT used annually throughout the world. A model suggests that maximum concentrations of DDT residues occurred in air in 1966 and will occur in the mixed layer of the oceans in 1971. The biota probably contains in total less than 1/30 of 1 year's production of DDT during the mid-1960's, a very small amount in proportion to the total potentially available. The reason for the biota's failure to absorb larger quantities and to be affected much more severely is unclear. The analysis suggests that mere good fortune has protected man and the rest of the biota from much higher concentrations, thus emphasizing the need to determine the details of the movement of DDT residues and other toxins through the biosphere and to move swiftly to bring world use of such toxins under rational control based on firm knowledge of local and worldwide cycles and hazards.

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