A DEGENERATE GLAND. PRINCE. 213 



an arteiy now appears, while the two urinary ducts or ureters 

 pass down each side of the swim-bladder posteriorly to the 

 urinary vesicle behind. In a young Callionyinits ^ inch (7 '8 

 mm.) long, the swim-bladder is capacious, and its walls thin, 

 and membranous ; but the floor is thick and its lining cells form 

 dense rugas not showing the clear mucoid character generally 

 distinguishing the internal cells of the organ in many species. 

 (Plate 21, tig. 10). The herring {Clupea Jiarengus) of the same 

 size, but no doubt very much older than Gobioids or Gadoids 

 such as those just described, exhibits a very capacious thin- 

 walled swim-bladder, the epithelial layer of cells being reduced 

 in thickness. The larva was -^j inch long (about 7 nun.) 

 and the notochord was extremely large, while the alimentary 

 canal was a narrow tube of very small capacity. Sections of a 

 Gurnard (Trigla) i\ in. (10 mm.) long, proved most interesting 

 as the walls show no less than five distinct layers and an 

 anterior portion with the usual mucus cell-lining was succeeded 

 further back by a blastema of deeply stained tissue in which 

 clear cells, possibly blood form-elements, are massed. Each 

 rounded clear cell showed a definite deep-stained nucleus. 

 This appearance suggests that a high vascularity is already 

 characteristic of the interior of the swim-bladder at this early 

 stage. The capacious front portion still, however, retains the 

 glandular features, the large clear lining cells, each with a 

 proximal nucleus, resting u[X)n a dense nucleated stratum, 

 outside being a complexly massed fibrous layer, external to 

 which is a fourth stratum of flattened cells, three or four cells 

 deep, the nuclei very marked, but much flattened, and outside all 

 is a thick connective tissue layer (the tunica externa) composed 

 of long interlaced fibres. Thus in the front part of the organ 

 there are no less than five layers, (Plate 21, fig. 12, a. b. c. d. e.) ; 

 but in the second portion the large-celled epithelium ceases, and 

 the four outside layers described are present. The pigmented 

 peritoneal membrane which encloses the organs of the abdominal 

 cavity appears below the swim-bladder anteriorly ; but further 



