»8 CLASS PISCES. 



before opening externally, but in Elasmobranchii the two ducts 

 open separately into the cloaca. The development of the Miil- 

 lerian duct is known only in Elasmobranchii. It there arises 

 in connection with the first establishment of the longitudinal 

 duct as an evagination of the parietal mesoderm of one of the 

 anterior nephrotomes, so that it at first consists simply of a 

 funnel-shaped opening of the longitudinal duct into the body 

 cavity. It soon, however, by a process of gradual shifting, 

 comes to open further and further back into that duct until it 

 acquires an independent opening into the cloaca. 



The female genital glands, which are, as is usual in Vertebrates, 

 specialised patches of the lining of the coelom, and of the unseg- 

 mented * portion of it called the splanchnocoel which persists 

 as the general body- cavity, dehisce their ova into the body 

 cavity, whence they escape by the Miillerian ducts — except 

 in Marsipobranchii and Teleostei. These exceptions however 

 are doubtful. In Marsipobranchs the genital pores by which 

 they escape may be Miillerian ducts, though it must be confessed 

 that there is not much to be said for so regarding them. In 

 Teleostei the ovaries are generally saccular and continued directly 

 into their ducts, but in some families they discharge into the body 

 cavity and the eggs are taken up by two funnel-shaped structures 

 which join each other and open behind the anus. It is quite 

 possible, though not definitely proved, that these funnels are 

 short Miillerian ducts, and that the ducts in the more usual con- 

 dition, in which they are continuous with the walls of the ovary, 

 are also Miillerian ducts, which have spread round the ovary or 

 fused with the edges of a peritoneal recess into which the ovary 

 has sunk. 



The male gonads also are specialised patches of the coelo- 

 mic lining, but the Marsipobranchii alone retain the primitive 

 condition of testis dehiscing into the general body cavity, 

 escape being made by genital pores of unknown homology. 



In all others the testis is continuous with its duct. In Teleos- 

 teans this continuity is very like the continuity found in the 

 female between the ovary and its duct, and the homology of the 

 male duct in these animals is not understood. It may be a per- 



* The contention which has been put forward in some quarters that 

 the gonads of Elasmobranchs arise from the segmented part of the coelora 

 cannot be seriously maintained. 



