  This is an updated version of ANT-0166.
  Generalizing results of Stroeker and Top we show that the 2-ranks of the
Tate-Shafarevich groups of the elliptic curves $y^2 = (x+k)(x^2+k^2)$ can
become arbitrarily large. We also present a conjecture on the rank of the
Selmer groups attached to rational 2-isogenies of elliptic curves.
