Primary Oocyte Arrest in Meiosis I
Meiosis is a process that involves two divisions, Meiosis I and Meiosis II, resulting in the formation of haploid gametes. In females, the process of meiosis is initiated during fetal development but remains arrested at certain stages until puberty. Let's delve into when the primary oocyte gets arrested in detail.

Formation of Primary Oocyte:
- During fetal development, oogonia undergo mitotic divisions to form primary oocytes.
- These primary oocytes are diploid cells containing 46 chromosomes.

Arrest at Prophase I:
- The primary oocytes enter Meiosis I during fetal development but get arrested at the diplotene stage of Prophase I.
- This arrest occurs in the ovaries before birth and remains until puberty.

Resumption of Meiosis at Puberty:
- At puberty, with the onset of menstrual cycles, some primary oocytes are stimulated to resume meiosis.
- One primary oocyte is selected each month for further development.
- This selected oocyte completes Meiosis I, resulting in the formation of a secondary oocyte and a polar body.

Further Arrest at Metaphase II:
- Following Meiosis I, the secondary oocyte enters Meiosis II but gets arrested at the metaphase stage.
- Completion of Meiosis II occurs only upon fertilization by a sperm.

Conclusion:
- The primary oocyte gets arrested at the diplotene stage of Prophase I during fetal development.
- It remains arrested until puberty when it resumes meiosis, leading to the formation of mature ovum upon fertilization.
- Understanding the process of oocyte arrest is crucial in reproductive biology and fertility studies.