Its cells then scrutinize nuclei and mend injuries, according to the theory. This idea bothered Sullivan. That's the most important time in development, so you'd think that it might be important to do it right. Sullivan wanted to see for himself what happened to faulty nuclei in developing embryos. He devised a way to look at chromosomes within dividing nuclei.
Normal chromosomes pair up to form rows, which facilitate their organization during division.
Sullivan was particularly interested in nuclei with jumbled, instead of neatly ordered, chromosomes. The nuclei containing these unruly chromosomes didn't divide properly, and lagged behind their neighbors. Sullivan saw a ruthless world, where nuclei that don't keep up get thrown out. In this way, the embryo maintains standards. A fruit fly embryo can lose as many as half its nuclei and still turn out right, Sullivan found.