ON THE DEVELOPMENT OF THE ELASMOBRANCH PISHES. 357 



duct, and then gradually extends backwards, so that the hind 

 end is still without a lumen, when the lumen of the front 

 end is of a considerable size. 



At the front knob the same alteration in the cells takes 

 place as in the rest of the duct, but the cells become deficient 

 on the side adjoining the pleuro-peritoneal cavity, so that an 

 opening is formed into the pleuro-peritoneal cavity, which 

 soon becomes of a considerable size. Soon after its first 

 formation, indeed, the opening becomes so large that it may 

 be met in from two to three consecutive sections if these are 

 very thin. 



Thus is formed the lumen of the Oviduct. The duct still, 

 at this age, ends behind without having become attached to 

 the cloaca, so that at this time the Oviduct is a canal closed 

 behind, but communicating in front by a large opening with 

 the pleuro-peritoneal cavity. 



It has during this time been travelling downwards, and is 

 now much nearer the pleuro-peritoneal cavity than the 

 epiblast. 



It may be well to point out that the mode of development 

 which I have described is really not very difi'erent from an 

 involution, and must, in fact, be only looked upon as a 

 modification of an involution. Many examples from all 

 classes in the animal kingdom could be selected to exemplify 

 how an involution may become simply a solid thickening. 

 In the Osseous fish nearly all the organs which are usually 

 formed by an involution have undergone this change in their 

 mode of development. I shall attempt to give reasons later 

 on for the solid form having been acquired in this particular 

 case of the Oviduct. 



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,p, w d) 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 formation), at numerous points with the 

 pleuro-peritoneal cavity. It is very probable that there is 

 one involution to each segment of the body between the front 

 and hind ends of the Oviduct. This duct is the Wolffian duct, 

 which thus, together with the Oviduct, is formed before the 

 appearance of the external gills. 



For a considerable period the front end of the Oviduct 



