Theodor Boveri said: "The nucleus does not divide, but it is divided".
The nucleus of a cell contains the genome and the genome is separated from the cytoplasm by the nucleus. But during cell division, in higher eukaryotes, the nuclear membrane breaks down or dissociates. The spindles separate the chromosomes and thus divide the genetic material. In lower eukaryotes, the nuclear membrane remains intact during cell division and the spindle fibers form within the membrane or they may penetrate the membrane without tearing it.
After that the nuclear membrane reform during telophase in higher eukaryotes.
So, it can be viewed as a passive structure that is divided by the cell's cytoskeleton. That is why Theodor Boveri said that statement and he meant this.