In human subjects, seminomas are the most common type of germinal tumor (30%) and the type most likely to produce a uniform population of cells. They almost never occur in infants, they peak in the fourth decade of the subject's life, somewhat later than the collective peak [3]. As in man, in the dog seminomas occur most frequently in older individuals, although their rarity in other domestic species may well be due merely to early castration of most other species [6]. In our case the owl monkey was also an older individual. While this tumor is often deadly to men [6], it is much less dangerous to the dog [6], and apparently to the owl monkey. It tends to spread slowly along the spermatic cord, but reasonably early removal is usually curative. Metastasis is rare [6]. A unique feature of some value in distinguishing the seminoma from other tumors encountered in the canine testicle is the rather frequent occurrence of small collections of cells, which are apparently lymphocytes, in the midst of the neoplastic cells [6]. In man, the typical seminoma has a homogeneous gray-white, lobulated cut surface, usually devoid of any hemorrhage or necrosis. In over half of the cases, the entire testis is replaced [3]. Microscopically, the typical seminoma, presumably derived from the proliferation of primary germ cells, presents sheets of uniform, socalled "seminoma cells" divided into poorly demarcated lobules by delicate septa of fibrous tissue. The classic "seminoma cell" is large and round to polyhedral and has a distinct cell membrane, a cleared or watery- appearing cytoplasm, and a large, central hyperchromatic nucleus with one or two prominent nucleoli [3]. Since the tumor resection 20 months has passed and the owl monkey's health condition is good and no hormonal or reproductive disorders have been observed. Seminomas could be expected to occur spontaneously probably in any primate species. More cases have to be studied to determine if similarities occur with the human disease.
