THE UROGENITAL SYSTEM — EXCRETION AND REPRODUCTION 569 



bryonic development when the embryo is sexually indifferent (Fig. 

 28.3). Its sex is determined genetically at the time of fertilization (p. 

 660), but early in development the embryo has the potentiality of 

 differentiating into either a male or a female, for the primordia of both 

 male and female duct systems are present. A pair of oviducts are pres- 

 ent, each one opening anteriorly into the coelom through a funnel- 

 shaped ostium and connecting posteriorly with the cloaca. The de- 

 veloping gonad, which is not recognizable as an ovary or a testis at first, 

 is adjacent to each mesonephros, and rete cords develop to connect 

 the gonad with some of the mesonephric tubules. Gametes can thus 

 pass through the rete cords, the mesonephric tubules and the Wolffian 

 duct. If the embryo differentiates into a male, the route through the 

 mesonephros materializes, and the embryonic oviduct disappears, leav- 

 ing at most a few traces. If the embryo differentiates into a female, 

 the route through the coelom and oviducts is used, the oviducts de- 

 velop further, and those parts of the male system not concerned with 

 excretion largely disappear. 



Male Vertebrates. In male frogs (Fig. 21.15) and other lower ver- 

 tebrates the rete cords become the vasa efferentia, which carry sperm 

 from the seminiferous tubules in the testis to the anterior part of the 

 kidney. The frog's kidney is an opisthonephros, but its anterior portion 

 develops from the embryonic mesonephros. Sperm pass through kidney 

 tubules into the Wolffian duct, which carries both sperm and urine to 

 the cloaca, though not at the same time. 



Higher vertebrates, such as man (Fig. 28.4), have metanephric 

 kidneys, and sperm pass from each testis to an epididymis, thence out a 

 vas deferens to the urethra. This, seemingly, is a different pattern, but 

 it is not as different as it first appears. Rete cords connect the semi- 

 niferous tubules with the epididymis and the epididymis represents 

 that part of the mesonephros that was associated embryonically with 

 the testis, together with a highly convoluted portion of the Wolffian 

 duct. The vas deferens represents the rest of the Wolffian duct, and 

 most of the urethra represents the ventral part of a divided cloaca. 

 Man thus utilizes passages homologous to those of a frog. 



Other differences between the male reproductive organs of lower 

 and higher vertebrates are correlated with differences in mode of re- 

 production. Frogs mate in the water and spray the sperm over the eggs 

 as they are discharged. Fertilization is external. This mating procedure 

 is perfectly satisfactory for species that mate in water, but the gametes 

 are too delicate for external fertilization in the terrestrial environment. 

 To accomplish internal fertilization, male mammals have a penis with 

 which to deposit the sperm in the female reproductive tract, and a 

 series of accessory sex glands that secrete a fluid in which the sperm 

 are carried. The penis develops around the urethra, and contains three 

 cavernous bodies composed of spongy erectile tissue. Venous spaces 

 within the erectile tissue become filled with blood during sexual excite- 

 ment, making the penis turgid and effective as a copulatory organ. The 

 accessory sex glands are a pair of seminal vesicles, which connect with 



