Final answer:
The egg nucleus is arrested in Metaphase II of meiosis before fertilization, a stage where the chromosomes are aligned at the cell equator, waiting for sperm arrival to complete cell division.
Step-by-step explanation:
The egg nucleus is arrested in the meiotic stage of Metaphase II before fertilization. This means that the egg cell halts its progress through meiosis at this phase until fertilization occurs. In Metaphase II, the chromosomes align at the cell equator, similarly to mitosis and unlike in Metaphase I where chromosomes pair up on the spindle. However, unlike in mitosis, these are not homologous chromosome pairs but rather individual duplicated chromosomes. The key difference between an egg in Metaphase II and a cell in mitotic metaphase is that the former is waiting to be fertilized before finishing its division process, while the latter proceeds directly to anaphase.
To clarify based on related information: during Prophase I, chromosomes condense, and the nuclear membrane breaks down. Metaphase I is characterized by homologous chromosomes lining up on the spindle. Anaphase I entails the separation of these paired chromosomes. Meanwhile, Metaphase II resembles mitotic metaphase, with a single line of individual chromosomes on the spindle ready for the second segregation, which occurs in Anaphase II.