212 THE SWIM-BLADDER OF FISHES 



after hatching, the organ retains its simple cellular character, a 

 single layer of large endodermal cells, not distinguishable from 

 the cells forming the wall of the general alimentary tract. 

 Soon a layer of flattened mesoderm cells (Plate 21, fig. 5, mes) 

 creeps round and invests the organ, becoming later the connective 

 tissue laj^er or tunica externa. The mesoderm cells rapidly 

 become greatly thickened, as the larva grows, and the lining 

 endoderm cells assume a swollen glandular character. Thus in 

 Trigla, the gurnard, when \ in. (6 nun.) long, the cells are so 

 enlarged as to crowd against each otiier, producing a very 

 irregular internal lining, forming thick rugfe, the nuclei of the 

 cells being, moreover, distally situated near the free ends of the 

 cells, the contents of the cells being clear and non-staining 

 (Plate 21, fig. 6). In a Gadoid, of the same length (6 mm.), 

 probably a pollack or coal lish, a similar appearance is presented > 

 but the nucleus of each swollen cell is central not distally 

 excentric in position and the tine fibrous layer outside, really 

 very much flattened and attenuated mesoderm cells, shows denie 

 black pigment (Plate 21, fig. 7, p) The presence of massed 

 pigment cells, forming a dark patch in the region of the swim- 

 bladder, is a marked feature in certain young gadoids (Plate 

 21, fig. 11). 



In sections of a young Gadoid th inch (6 '5 mm.) long, 

 probably a young cod a pollack I cannot now say which, for 

 these allied species are practically identical in internal structure 

 in larval life, the pigment layer is now more distinctly separated, 

 and the nuclei in the large clear cells lining the swim-bladder 

 are proximal and nearer to the thickened connective tissue layer. 

 (Plate 21, fig. 8). In slightly older post-larval Gadoids say f 

 inch (92 mm.) long the cavit}^ of the swim-bladder has greatly 

 increased, the large mucus cells, with proximal nuclei are more 

 regularly arranged (Plate 21, fig. 9), but a posterior portion of 

 the organ is now marked off, with thin walls and showing no 

 mucus-cell lining, indeed a dense thickened tubular section 

 separates the anterior and posterior parts, in the wall of which 



