the median axis. It is a somewhat more troublesome matter, however, to bring these conôÙfiôÙgurôÙaôÙtions into relation with a ãlaw of growth,ã as was so easily done in the case of the circular figure: in other words, to {565} formulate a law of acceleration according to which points starting from the origin O, and moving along radial lines, would all lie, at any future epoch, on an ellipse passing through O; and this calculation we need not enter into.
All that we are immediately concerned with is the simple fact that where a velocity, such as our rate of growth, varies with its direction,ãvaries that is to say as a function of the angular divergence from a certain axis,ãthen, in a certain simple case, we get lines of growth laid down as a system of coaxial circles, and, when the function is a more complex one, as a system of ellipses or of other