We have measured the concentrations of dry matter in mitotic apparatuses (MA) in vivo and after isolation from the same cell type. The isolation medium was hexylene glycol plus buffer. The MA were from sea-urchin zygotes (Echinus esculentus Linn, and Psammechinus miliaris Gmelin), and measurements were made using interference microscopy. MA as isolated have much lower concentrations of dry matter than do MA in vivo. The dry mass concentrations of isolated MA vary with the pH of the isolation

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... ium, ranging from about 20 % of the in vivo concentration (at pH 7-3) to about 60 % of the in vivo concentration (atpH 5 -3 ). The isolated MA were further characterized. Evidence is presented which suggests that non-specific cytoplasmic material adsorbs to MA, and thus that at least some of the material in isolated MA is not derived from in vivo MA. Some MA components are apparently changed during the isolation procedure: MA lysed in low pH (high mass) medium and quickly transferred to high pH (low mass) medium have higher concentrations of dry matter than do MA lysed in high pH medium. The isolation media as generally used do not have enough buffering capacity: the pH changes after the isolation. In this report we describe measurements of the concentration of dry matter in in vivo MA and in MA isolated from the same cell type (i.e. fertilized sea-urchin eggs). The data confirm that MA do indeed lose a large portion of their dry matter during the isolation procedure, and show that, as described briefly elsewhere (Forer & Goldman, 1969) , the concentration of dry matter present in isolated MA depends on the pH of the isolation medium. The data further suggest that at least some of the material present in isolated MA has properties which differ from in vivo MA, and that some of the material in isolated MA comes from the cytoplasm and not from in vivo MA. METHODS Animals and gametes Some sea urchins (Echinus esculentus Linn.) were supplied by the Scottish Marine Biological Association Laboratories (Millport), and used either in Millport, Glasgow or Cambridge. Other sea urchins (PsammeMnus miliaris Gmelin) were collected at Hunstanton. Gametes were extruded from the aboral pores after animals were injected with o-6 M KC1 (Harvey, 1956) . Eggs were collected in filtered sea water, and prior to fertilization were washed in several changes of filtered sea water. Testes were removed from male sea urchins and stored 'dry' in Petri dishes at 4 °C. • Measurements of' MA' refer to measurements at the spindle equator, while measurements of ' asters' refer to measurements at the centres of asters.