ABSTRACT
The testes of birds are intra-abdominal, and, unlike in most mammals, they do not migrate from their site of embryological origin. They are, thus, closely related, topographically, to the kidneys. As in mammals, birds have two testes, one on either side of the midline, bordering the aorta and caudal vena cava, laterally. They are attached from their dorso-medial borders to the dorsal abdominal wall by a short mesorchium, and as the kidneys with which they are related embryologically, they are largely retroperitoneal, and ventral to the vertebral column. Topographically, the cranial poles (extremitates craniales) of the testes lie close to the ventral border of the lungs, while their caudal poles (extremitates caudales) lie cranio-ventral to the cranial divisions of the kidneys (Nickel et al. 1977). The dorsomedial aspect of the testis attaches to the relatively small epididymis. The ductus deferens runs distally from the caudal border of the epididymis, toward the cloaca, into which it opens. There are no known accessory sex organs or glands in birds that are either homologous or analogous to those found in mammals. Mammalian terminologies have often been erroneously applied to certain structural modifications in birds, such as seminal vesicle in passerine birds, and the ampulla of the ductus deferens. These have to be understood for what they are, structurally and functionally, as segmental convolutions or enlargements, respectively, of the ductus deferens.
