214 THE SWIM-BLADDER OF FISHES 



back it ceases, and the organ lies directly upon the liver and 

 alimentary canal. The peritoneum never indeed surrounds the 

 swim-bladder, at any time during the life of the fish. If an 

 embryologist, or an anatomist, were for the first time confronted 

 with an organ having the characteristics of the swim-bladder 

 in fishes during their early larval life, he would, without 

 hesitation, pronounce it a gland. The salivary glands, for 

 instance, in the human subject develop in the six-week embryo 

 as a protrusion of the deeper epithelial layer of the mucous 

 coat of the oral cavity becoming hollow after its protrusion and 

 developing follicles, in which occur large transparent cells, 

 each with an excentric nucleus stained by carmine while 

 the surrounding mass of cell-substance remains clear and 

 unstained. Other cells, called " peripheric," occur in the follicles 

 which are not mucous, but albuminous and stain covnpletely. 

 Dr. W. B. Carpenter said (No. 3, p. 1 32), " It is believed that 

 the albuminous cells during the period of rest of the gland 

 gradually become metamorphosed and develop into the mucin- 

 holding cells." I do not wish to attach too much importance 

 to the circumstance ; but in one specimen of a cod, Hi mm. 

 (^1 inch) long, the anterior portion of the swim-bladder was 

 lined, not by large clear cells, with a nucleus alone staining by 

 carmine ; tut a dense deep stainino; mass of reduced agglomerated 

 cells as though the large cells had become metamorpho ed into 

 something like the small dark granular cells which fill the 

 salivary acinus during the period of active secretion. The 

 swim-bladder, in its earliest condition, may be compared to a 

 large gland, not compound and complexly developed like the 

 liver ; but a simple sac or huge follicle, its fundus or distal 

 region lined by large epithelial cells, and leading into a non- 

 glandular second j)ortion, which may indeed be regarded as 

 having subserved a storage function like the gall-bladder in 

 that great secreting gland, the liver, and finally leading by a 

 duct to the opening into the gullet which may or may not have 

 a sphincter muscle. This duct may in a large number of species 



