الفهرس 
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Abstract
The fish has economic importance especially the reproductive system so the present study was conducted to investigate the histological structure of 120 fish were utilized.
Testes
The testis of Oreochromis niloticus is surrounded by tunica albuginea and comprises seminiferous lobules containing Sertoli cells and different stage of spermatogenic cells. These stages are; undifferentiated, primary and secondary spermatogonia, primary and secondary spermatocytes, spermatids and spermatozoa. The testis of Oreochromis niloticus shows seasonal histological changes. During winter season, the testicular lobules decrease in size and all of spermatogenic cells are present but spermatogonia increase in number to renew the germ cells, the testes during winter are in resting or inactive season. During spring and summer seasons, the testicular lobules increase in size where they reach a maximum size during summer as they become distended with the different developmental stages. Some lobules are fully distended with spermatozoa and others appear empty as they spermiate. The spring and summer can be known as spawning season. During autumn season, the testicular activity begins to decrease than those of spawning season but, some testicular lobules still contain residues of undischarged spermatozoa, while other lobules appeared empty, the autumn can be known as postspawning season.
Ovary
The ovary of Oreochromis niloticus is surrounded by tunica albuginea and contains oogonia and follicles in different developmental stages. These stages are; chromatin-nucleolus stage, perinuclear stage, cortical alveolar stage, vitellogenic stage and mature stage. Some atretic follicles are also present. The ovary of Oreochromis niloticus shows also seasonal histological changes. During winter season, the ovary shows a maximum thickness of tunica albuginea with predominance of chromatin- nucleolus and perinucleolus oocytes, the ovary in winter is considered in resting or inactive season. During spring and summer seasons (spawning season), the tunica albuginea decreases in thickness with predominant cortical alveolar , vitellogenic and mature stage where the ovary is in spawning season and becomes ready to spawn and ovulate. During autumn (post-spawning) season, the tunica albuginea is relatively thicker with predominance of atretic follicle which indicates the spawned fish.