What is the difference between a primary oocyte and a secondary oocyte?

Primary and secondary oocytes

Oogenesis is the process of formation of ovum in the female reproductive system.

The primary oocyte is formed from the oogonia whereas the secondary oocyte is formed when the primary oocyte undergoes meiosis.

Cell involved in division

Primary oocytes are formed from the oogonia by the mitotic division. Secondary oocytes are formed from the primary oocytes by first meiotic division.

Period of development

Primary oocytes are formed during the embryonic development and the secondary oocytes are developed during puberty.

Number of chromosomes present

Primary oocytes contain $46$ chromosomes. Secondary oocytes contain $23$ chromosomes.

Ploidy

Primary oocytes are diploid ($2n$) and secondary oocytes are haploid ($n$).

Summary

$\begin{array}{ccc}\text{Criteria}& \text{Primary oocyte}& \text{Secondary oocyte}\\ \text{Cells involved in division}& \text{Oogonia}& \text{Primary oocyte}\\ \text{Period of development}& \text{Develops during embryonic stage}& \text{Develops during pubert}\\ \text{Number of chromosomes present}& 46& 23\\ \text{Ploidy}& 2n& n\end{array}$