Although tracking down the Leydig gland in the writings of Franz Leydig has been a challenge, there are plenty of references in the modern biology literature. As noted in the online biography of Franz Leydig (Complete Dictionary of Scientific Biography):
Leydig is also known for the discovery of the gland of Leydig (1892), a portion of the mesonephros in vertebrates, of which the secretions are thought to stimulate the movement of spermatozoa … (Leydig, 2008, Internet - see References)
However, mentions of the Leydig gland appear to be almost entirely in articles about fishes. Thus, the Leydig gland appears to function as an accessory organ for the reproductive tract, much as the prostate, Cowper's gland, and seminal vesicle do in humans. Below is a sampling from research articles to provide a sense of the modern literature:
In the elasmobranches [e.g., great white shark], sperm released from the ampullae into the efferent canals pass through a mass of coiled glandular tubules (gland of Leydig) which are derived from the anterior nonurinary portion of the mesonephrous. Sperm contained in the secretions of the Leydig gland then pass into an expansion of the vas deferens known as the ampulla. (Hoar and Randall, 1969, p. 15)
The testes of the male chimaerid lie far forward in the body cavity. The efferent duct differ in that they are more numerous and form an anastomosing network in the mesorchium. The epididymis is well-developed as is the Leydig’s gland. (Khanna and Yadav, 2004, p. 166)
In the Greenland shark, Laemargus borealis, the Wolffian duct is absorbed in the adult. The sexual part of the opisthonephros of elasmobranches functions as the epididymis. This leads into the ductus deferens, which may at its caudal end have diverticula that serve as sperm storage organs (sperm sacs), or the ductus deferens may have a wide ampulla. The cranial part of the kidney forms a Leydig gland which secretes the seminal fluid. This Leydig gland is not homologous with the Leydig cells (which are sometimes called Leydig gland) of the higher vertebrates. (Khanna, 2004, p. 66)
Both the epididymis and ductus deferens receive a viscous fluid produced by the adjacent Leydig gland. In sexually immature specimens the ductus is a thin, straight tube, but in sexually mature males it is coiled and covers most of the kidney. The two broad seminal vesicles receive no ducts from the Leydig gland. Each seminal vesicle has a lateral dilation termed the sperm sac. This is a misnomer because these structures do not store sperm but are actually Marshall’s alkaline gland…. Three main types of particulate material have been identified as occupying the male genital ducts Heterodontus portusjacksoni, namely Sertoli cell bodies, Sertoli cell cytoplasts, and Leydig gland bodies…. Leydig gland bodies are large, eosinophilic, non-membrane-bound secretions of Leydig glands. (Hamlett, 1999, p. 450)
Jones and Lin (1993) conclude that Leydig gland secretions are the main source of the increase in protein concentration of the luminal fluid in the ductus deferens in Herodontus…. These data suggest that the epididymis and Leydig gland secretions play important roles in ion and water transport, protein secretion, and maturation of spermatozoa. (Hamlett, 1999, p. 453)
The Mesonephric Ducts. The conditions in the elasmobranchs have been regarded as very primitive. In them (and to some extent in some of the amphibia), when the mesonephros develops, the pro-nephric duct divides longitudinally from its hinder end as far forward as the anterior end of the Wolffian body. Of the two ducts thus formed (fig. 321, A), one, the Wolffian (Leydig's) duct, remains connected with the tubules of the mesonephros and forms its excretory canal. (Kingsley, 1912, p. 316)
Each testis is connected with the anterior genital part of the kidney of its side by several fine tubules called vasa efferentia. These come out of the anterior end of the testis, run in the mesorchium, enter the anterior part of the genital kidney and open into its tubules. These nonexcretory kidney tubules open into the Wolffian duct which now is called the vas deferens. The vas deferens is a long, narrow and greatly coiled tube. It runs on the ventral surface of the anterior part of the kidney. [The anterior part of the kidney penetrated by the vasa efferentia is called the epididymis and the remaining part of the nonexcretory kidney along which the vas deferens runs is called the organ of Leydig or Leydig’s gland. The Leydig’s gland secretes nourishing material for the spermatozoa]. (Bhatnagar and Bansai, 2008, p. 190)
Spermatophores produced by elasmobranches are the result of secretory activities of male accessory ducts. After spermiation, sperm pass through vasa efferentia and enter the coiled tubules of the Leydig gland, which is derived from the anterior portion of the mesonephric kidney. It is not steroidogenic and should not be confused with the Leydig cells. Sperm and secretions of the Leydig gland pass on to an expanded region of the vas deferens known as the ampulla. Here the sperm are consolidated and receive additional secretory material to form complex spermatophores typical for each species. (Norris, 2007, p. 383)
Unusual features of the reproductive system include an epigonal organ in males and females. It contains lymphoid and hemopoietic tissue. Leydig's gland, a modified region of the kidney, produces seminal fluid. In some species, sperm passing through the vas deferens, is enclosed in spermatophores. (Wourms, 1977, p. 379)
In transit through the ducts, spermatozoa undergo modification by secretions of the extratesticular ducts and associated glands, i.e., Leydig gland. In mature animals, the anterior portion of the mesonephros is specialized as the Leydig gland that connects to both the epididymis and ductus deferens and elaborates seminal fluid and matrix that contribute to the spermatophore or spermatozeugmata, depending on the species. Leydig gland epithelium is simple columnar with secretory and ciliated cells. Secretory cells have periodic acid-Schiff positive (PAS+) apical secretory granules. In the holocephalan elephant fish, Callorhynchus milii, sperm and Sertoli cell fragments enter the first major extratesticular duct, the epididymis. In the epididymis, spermatozoa are initially present as individual sperm but soon begin to laterally associate so that they are aligned head-to-head. The epididymis is a highly convoluted tubule with a small bore lumen and an epithelium consisting of scant ciliated and relatively more secretory cells. Secretory activity of both the Leydig gland and epididymis contribute to the nascent spermatophores, which begin as gel-like aggregations of secretory product in which sperm are embedded. Fully formed spermatophores occur in the ductus. The simple columnar epithelium has both ciliated and secretory cells. (Hamlett et al, 2003, p. 111)
Bhatnagar, M. C. and Bansai, G. (2008). Chordata. Meerut, India: Krishna House.
Hamlett, W. C. (1999). Sharks, Skates, and Rays: The Biology of Elasmobranch Fishes.
Hamlett, W. C., Reardon, M., Clark, & Walker, J. T. (2003). Ultrastructure of sperm storage and male genital ducts in a male holocephalan, the elephant fish, Callorhynchus milii. Journal of Experimental Zoology. 292(2):111-28.
Hoar, W. S. and Randall, D. J. (1969). Fish Physiology: Volume III (Reproduction and Growth. New York: Academic Press.
Khanna, D. R. (2004). Morphogenesis. New Delhi, India: Discovery Publishing House.
Khanna, D. R. and Yadav, P. R. (2004). Biology of Fishes. New Delhi, India: Discovery Publishing House.
Kingsley, J. S. (1912). Comparative Anatomy of the Vertebates. Philadelphia: P. Blaiston’s Son & Co.
"Leydig, Franz von." Complete Dictionary of Scientific Biography. 2008. Retrieved May 30, 2013 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-2830902604.html
Norris, D. O. (2007). Vertebrate Endocrinology. Burlington, MA: Elsevier Academic Press.
Wourms, J. P. (1977). American Zoology. (1977) 17 (2): 379-410.