1. The number of spermatozoa in a sample of sea-urchin semen (Echinus esculentus) can be accurately estimated by measuring the amount of light scattered and absorbed by a subsample composed of semen diluted with sea water. A ‘Spekker" photoelectric absorptiometer was used for the measurements, with a cell 1 cm. long.

2. The number N of spermatozoa in a sample of sea-urchin semen was found to be given by the equation N=8.0918x1010Y'±(1.590x109Y')where Y' =0.3010-log10y' and y' = absorptiometer reading, when the original sample was diluted 200 times and the absorptiometer set to 2.000 for the blank.

3. The light-scattering and light-absorbing properties of washed and unwashed suspensions were found to be virtually identical.

4. The number of spermatozoa in a sample of bull semen was found to be given by the equation N=3.3515x109Y'±(5.283x107Y'),when the original sample was diluted 30 times and the absorptiometer set to 2.000 for the blank.

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