DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 923 



is most distant from the free edge of the cell. In the later post-larval stages of the 

 gurnard further changes occur, the deep glandular epithelium of the anterior part of the 

 swim-bladder enclosing a narrow lumen. A little further back the thick glandular 

 epithelium is continued into a lateral blastema of deeply stained tissue on each side. 

 The cells are, however, altered in form, being spherical with a definite nucleus, and the 

 cell-contents are now clear. The lumen of the bladder becomes posteriorly narrowed in the 

 form of a neck, and pulpy globular cells, with deeply stained (glandular ?) contents take 

 the place of the cubical epithelial cells of the fore part of the sac. These rounded cells 

 form a superficially broad girdle. Behind the narrow neck the bladder expands again, but 

 its walls are thinner and the layers distinguishable are fewer. Thus, in the anterior part, 

 outside the greatly thickened mucous lining, a dense nucleated stratum occurs, which rests 

 upon a striated fibrous layer, bounded externally by a very thin nucleated stratum — the 

 nuclei being much flattened and elongated, and lying two or three deep. An outer tunic of 

 thick connective-tissue circumscribes the bladder, and this rests in the anterior part upon 

 the pigmented peritoneum. Posteriorly, the liver and intestine are in contact with the 

 external connective tunic. The four layers just described, with the exception of the deep 

 internal mucous layer, continue into the second part of the bladder, but are much thinner. 

 The great bulk of the liver and the pronephric augmentation are probably influential 

 in the shifting backward of the swim-bladder. As shown in the earlier part of this 

 paper, the swim-bladder is a protrusion from the embryonic oesophagus ; but the 

 lengthening of the gullet, the enlargement of the stomach and intestine, produce such 

 changes in the disposition of the abdominal viscera as greatly to disturb the primitive 

 relations of the various organs. Professor Cleland, in a valuable note* points out that 

 the origin of the swim-bladder as a thoracic evagination must determine the regions 

 of the alimentary tract (e.g., pharynx, stomach, and intestine) ; but it has to be borne in 

 mind that, when the evagination takes place, the tract is very short, and the cystic 

 duct is pushed out of the ventral wall in such close proximity to the outgrowth of the 

 swim-bladder as to be included in the same section of the embryo, if cut in a slightly 

 oblique plane, — the duodenum and pharynx in the early stage being separated by a very 

 short interval. It is possible, therefore, that in the elongation and differentiation of the 

 parts of the alimentary canal, the point of origin for the swim-bladder may, in post-larval 

 and still more in adult stages of different species, be found in parts which do not perform 

 the same physiological function. The position stated by Professor Cleland is not, how- 

 ever, affected by this consideration, and the part called stomach in such a form as Clupea 

 must be morphologically — if not functionally — pharyngeal. 



Liver. — The liver in Anarrhichas appears on both sides of the fish posteriorly, but 

 in the salmon it is best seen on the right side. Consequently the arrangement of the 

 blood-vessels which pass through it for the supply of the yolk- mass diverge considerably 

 in the two species. The position of the liver is seen in the outlines of the right and left 

 sides (PI. XX. figs. 2 and 4). 



* Memoirs in Anatomy, 1889. 



