Physiology of Finfish and Shellfish (2+1)

3.1.10. Oogenesis

Oogenesis is the earliest stages of the development of the female germ cells are similar to those seen in the initial stages of spermatogenesis. The germ cells first undergo proliferation via a series of mitotic divisions and then they develop to become primary oocytes. There is a period of growth and a meiotic reduction division. The final reduction division with expulsion of the second polar body does not occur until after fertilization, when the egg has been activated by the sperm. 

The growth of the oocytes varies from species to species with oocyte enlargement being caused largely by the accumulation of yolk. Yolk accumulation occurs during the developmental period known as vitellogenesis. The oocyte enlargement may be considerable for eg, the oocytes of salmonids increasing in diameter from 20-50 µm to 4-5 mm during the course of development.