The undescended testis is at increased risk of testicular torsion. Germ cells normally occur inside the gonads (ovary and testis). The testes in birds are generally asymmetric with most birds having a larger left testis. A major risk factor for the development of testis cancer is cryptorchidism (undescended testicles). It is also seen with torsion of the testis and with some testicular tumors. The right side of the anterior testis has a branched, tubular ovary. Adults possess both testis and ovary in different levels of development. It is located within a highly vascular connective tissue in the mediastinum testis. With color Doppler ultrasonography, the vascularity of the undescended testis is poor. The cremaster muscle is a muscle that covers the testis and the spermatic cord. ROCK1 is mainly expressed in the lung, liver, spleen, kidney and testis. A secondary hydrocele is secondary to either inflammation or a neoplasm in the testis. Both insert into the tunica vaginalis underneath the testis. The homologous organ in the male is the rudimentary appendix testis. SRY acts on gene SOX9 which drives Sertoli cell formation and testis differentiation.