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 
guard 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 cesophagus; 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 (Pl. XX. figs. 2 and 4). 

* Memoirs in Anatomy, 1889. 
