9o GENITAL DUCTS 



germ-cells sink below the coelomic epithelium, and give rise to a 

 system of canals which generally join to a longitudinal testis-canal. 

 From this runs the network of transverse canals, vasa efferentia ; 

 and these usually join again near the base of the mesonephros 

 (Fig. 56) to a longitudinal canal, into which open the mesonephric 

 tubules leading to the Malpighian capsules. This plan of structure 

 is found throughout the Gnathostomes, with slight modifications, 

 being only secondarily altered in some specialised groups (Teleo- 

 stomes, p. 364 ; Anura). Thus, the spermatozoa are never shed 

 into the body-cavity in the Gnathostomes. 



The network of canals joining the testis to the mesonephros is 

 derived from three sources in ontogeny : the testis canals, the fold 

 of coelomic epithelium closing in the longitudinal canal (and vasa 

 efferentia) near the base of the testis, and the kidney tubules which 

 open into it. These three factors may contribute in varying 

 proportions. The continuous system of canals in the male is, then, 

 formed from a longitudinal chamber of the coelom closed off near 

 the base of the genital ridge, into which open, on the one hand, the 

 testicular canals, and, on the other, the mesonephric funnels (Fig. 

 55, and D). Rudiments of these ducts may be present in the 

 female (Spengel [414], Mihalkovics). 



The female sex in the Gnathostomata presents a more primitive 

 condition (again with the exception of some Teleostomes, which 

 will be dealt with later, p. 3C7). The ova are shed into the 

 abdominal coelom, and are carried out by the open -funnelled 

 Mullerian ducts (Figs. 55, E ; 56, C). 



Much controversy has taken place concerning the exact homology 

 of the genital ducts ; into the details of the question we need not 

 enter here. Putting aside for the present the Teleostomes (p. 

 367), it may be pointed out that both the Mullerian and the 

 Wolffian duct are present in both sexes ; and that, while the latter 

 is clearly the mesonephric duct, the real difficulty lies in determining 

 the homology of the former. 



That the oviducts in the Elasmobranch, the Dipnoan, the 

 Amphibian, and the Amniote are homologous structures cannot be 

 doubted on the evidence of comparative anatomy ; the position of 

 the ostium abdominale, the course of the duct running along the 

 abdominal wall outside the mesonephros, the posterior opening into 

 the cloaca — these and other characters are essentially similar in all 

 the Gnathostomes mentioned above. Yet on the uncertain evidence 

 of embryology this conclusion is sometimes denied. 



It has been clearly demonstrated (Balfour [27], Kabl [337]) 

 that the archinephric duct, in the Elasmobranch embryo of both 

 sexes, becomes split into two from before backwards, in such a 

 way that the pronephric tubules remain connected with one of 

 the resulting tubes (the ' pronephric ' or Mullerian duct), and 



