100 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



At about the time when a lumen appears in the Oviduct the 

 first traces of the Wolffian duct become visible. 



At intervals along the whole length, between the front and 

 hind ends of the Oviduct, involutions arise from the pleuro- 

 peritoneal cavity (fig. 12, a,pwd] on the inside (nearer the 

 middle line) of the Oviduct. The upper ends of these numerous 

 involutions unite together and form a string of cells, at first 

 solid, but very soon acquiring a lumen, and becoming a duct 

 which communicates (as it clearly must from its mode of for- 

 mation), at numerous points with the pleuro-peritoneal cavity. 

 It is very probable that there is one involution to each seg- 

 ment of the body between the front and hind ends of the 

 Oviduct. This duct is the Wolffian duct, which thus, toge- 

 ther with the Oviduct, is formed before the appearance of the 

 external gills. 



For a considerable period the front end of the Oviduct does 

 not undergo important changes ; the hind end, however, comes 

 into connection with the extreme end of the alimentary canal. 

 The two Oviducts do not open together into the cloaca, though, 

 as my sections prove, their openings are very close together. 

 The whole Oviduct, as might be expected, shares in the general 

 growth, and its lumen becomes in both sexes very considerably 

 greater than it was before. 



It is difficult to define the period at which I find these 

 changes accomplished without giving drawings of the whole 

 embryo. The stage is one considerably after the external 

 gills have appeared, but before the period at which the growth 

 of the olfactory bulbs renders the head of an elongated shape. 



During the same period the Wolffian duct has undergone 

 most important changes. It has commenced to bud off diverti- 

 cula, which subsequently become the tubules of the Wolffian 

 body (vide fig. 13, wd}. I am fairly satisfied that the tubules 

 are really budded off, and are not formed independently in the 

 mesoblast. The Dog-fish agrees so far with Birds, where I have 

 also no doubt the tubules of the Wolffian body are formed as 

 diverticula from the Wolffian duct. 



The Wolffian ducts have also become much longer than the 

 Oviduct, and are now found behind the anus, though they do not 

 extend as far forward as does the Oviduct. 



