Fig. 9. Transepithelial flux of fluorescein as a function of bathing saline fluorescein concentration for isolated Drosophila tubules set up in Ramsay assays. Each point shows the mean ± S.E.M. for N=5 tubules. The solid line represents the fit to the Michaelis–Menten equation by non-linear regression analysis. The inset shows the ratio of the concentration of fluorescein in the secreted fluid to that in the bathing medium (S/M) as a function of bathing medium fluorescein concentration.