DEVELOPMENT OF THE ELASMOBRANCH FISHES. 99 



peritoneal cavity, always remaining attached to the intermediate 

 cell mass. 



At about the time when five visceral clefts are present the 

 Oviduct first begins to get a lumen and to open at its front end 

 into the pleuro-peritoneal cavity. The cells of the rod are first 

 of all arranged in an irregular manner, but gradually become 

 columnar and acquire a radiating arrangement around a central 

 point. At this point, where the ends of all the cells meet, a 

 very small hole appears, which gradually grows larger and 

 becomes the cavity of the duct (fig. 12, o v). The hole first 

 makes its appearance at the anterior end of the 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 for- 

 mation, 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 different from an invo- 

 lution, 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. 



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