Right.
But then, again, a beginning and an end are the limits of everything?
Certainly.
Then the one, having neither beginning nor end, is unlimited?
Yes, unlimited.
And therefore formless; for it cannot partake either of round or straight.
But why?
Why, because the round is that of which all the extreme points are equidistant from the centre?
Yes.
And the straight is that of which the centre intercepts the view of the extremes?