Reproduction 283 



The gonads find outlet by way of ducts which arise in relation to 

 the kidneys. The seminiferous tubules of the testis (Fig. 195) acquire 

 connection with the neighboring mesonephric tubules and thereby gain 

 exit by way of the Wolffian duct which therefore, in fishes and am- 

 phibians, serves as a urinogenital duct. In other vertebrates the adult 

 male retains, in the epididymis, that part of the embryonic meso- 

 nephros which provided connection between the testis and the Wolff- 

 ian duct. With metanephros and ureter serving the urinary function, 

 the Wolffian duct is left as a vas deferens or sperm-duct only. 



The oviducts in elasmobranchs and probably some amphibians 

 arise by longitudinal splitting of the pronephric duct, one portion of it 

 serving thereafter as the mesonephric duct while the other portion 

 (Mullerian duct) acquires, by fusion of several pronephric nephro- 

 stomes, a wide anterior opening into the coelom in the vicinity of the 

 ovary. In other vertebrates, a Mullerian duct develops as a fold of 

 peritoneum (Fig. 87, m) closely parallel to the Wolffian duct but inde- 

 pendent of it. The Mullerian duct develops alike for a time in both 

 male and female embryos, but only vestiges of it persist in the adult 

 male. 



Mesenchyme. Reference has been made (p. 278) to the fact that 

 certain regions of the mesodermal somite, the sclerotome and the der- 

 matome, are the source of cellular material which becomes detached 

 from the somite and aggregates in the spaces between the somite and 

 neighboring organs or layers, where it produces skeletal, connective, 

 and integumentary tissues. This secondary mesoderm ("derm" imply- 

 ing a sheet or layer), being usually not disposed in definite layers, is 

 called mesenchyme. But the somite is not the only source of mesen- 

 chyme. Quantities of it are produced in most or all regions of the 

 embryo. 



Beyond question, most of the mesenchyme comes from the meso- 

 derm. The parietal and visceral layers of the hypomere are a prolific 

 source of it, numerous cells becoming detached from the outer (next 

 to the ectoderm) surface of the parietal layer and from the inner (next 

 to the endoderm) surface of the visceral layer. Also the endoderm con- 

 tributes to the mesenchyme which accumulates between the enteric 

 wall and the adjacent layers of mesoderm. The ectoderm plays a minor 

 part, but evidence has been found indicating that mesenchyme of ecto- 

 dermal origin, "mesectoderm," participates in the development of 

 parts of the skeleton of the pharyngeal region. 



Mesenchyme spreads from its place of origin and eventually is 

 found in all parts of the embryo. Although late in origin, its importance 

 is by no means secondary. Chief among its derivatives are the follow- 

 ing materials and structures. 



