GAS-SECRETING TISSUES 



335 



of this nucleus is a sphere of darker staining cytoplasm that appears 

 much like the centrosphere of some spermatogonia. Around this sphere 

 is a zone of lighter 

 staining substance 

 that is yet darker 

 than the cytoplasm, 

 and whose ends are 

 expanded on the side 

 Jthes, fro ra the nu- 

 cleus into two wing- 

 like processes that 

 reach almost to the 

 cell-wall. The secre- 

 tion appears in the 

 form of fine granules 

 that swell and finally 

 are transformed into 

 the gas near the pe- 

 riphery of the cell. 



Gas secretion in 

 the teleost fish, Gadus 

 morhua, and others. 

 Many of the teleost 

 fishes possess a swim-bladder that serves to reduce their specific grav- 

 ity by secreting and containing a gas mixture. The gas is secreted by 

 the epithelial lining of the organ. 



As the swim-bladder is formed by an embryonic invagination of the 

 intestinal tract, this gas epithelium is genetically related to the respiratory 

 cells of the vertebrate lung, whether the swim-bladder and the lung are 

 the same or different invaginations, phylogenetically, of this region or not. 



But while the respiratory cells passively allowed various gases to 

 diffuse themselves through their cytoplasm, this epithelium of the swim- 

 bladder, as has been said, secretes it into a chamber that is under a 

 mechanical pressure, and perhaps a chemical condition of resistance 

 as well. 



The epithelium sometimes covers the entire inner surface of ftie swim- 

 bladder, while in our subject, the cod, only that portion on a limited area 

 of this lining epithelium is so used. This smaller part, however, is 

 specialized into a condition of greater efficiency by, first, an amplification 

 of its surface through numerous tubular and folding invaginations and, 

 secondly, by an increase in the size and thickness of the secreting cells 

 themselves (Fig. 299). This is accompanied by an increased peripheral 

 blood supply that is pushed up in capillary loops into the regions between 



FIG. 298. Part of a gas-secreting cell from the siphonophore 

 medusa, Rhizophyza filiformis. nu., nucleus showing a dark 

 and crowded chromatin pattern; x, unknown centrosphere- 

 like body. (After K. C. SCHNEIDER.) 



