A method for visualizing the chromosomes of the second polar body of the mouse egg.

The second polar body, extruded from the egg at the time of fertilization, is in principle capable of reactivation, as shown by its successful reincorporation into the egg, for example as a result of hypotonic treatment (Opas, 1977). Efforts to produce chromosome preparations from the second polar body have so far, however, been unsuccessful: during normal development, if it divides at all it does so unpredictably, out of synchrony with the embryo. C. E. Ford suggested to us that it might be possible to induce the second polar body to enter mitosis by injecting it into a fertilized egg, and this we have now confirmed in mice.