334 



HISTOLOGY 



chr. 



where the basal portion of the cell begins to widen, about one third of 

 the distance from the base. The secretion first appears as a group of 



small granules immediately distad of 

 the nucleus, and these granules move 

 toward the distal end of the cell, 

 where they swell and become filled 

 with the gas. The large gas bubbles 

 rupture the cell-wall and break into 

 the gas chamber to supply it. The 

 whole membrane with its epithelium is 

 thrown into a series of parallel folds 

 of moderate depth. These folds be- 

 come of lesser depth on either side of 

 a central area. 



A short account of the gas cells of 

 another siphonophore, Physophora hy- 

 drostatica, which occur in a more 

 highly specialized organ, should be 

 considered here. 



The gas cells in this form are 

 found on a membrane homologous to 

 that which bears them in Physalia. 

 But all of the epithelial cells on this 

 membrane are not developed into the 

 gas cells. Most of them are a simple 

 cell representing the secondary ecto- 

 derm (from which the gas cells also 

 arise) in its simplest form. Only, in- 

 stead of both sorts of cells forming a 

 single row, the undifferentiated cells 

 form a thick, many-layered mass in 

 which the gas cells are placed sparingly 

 and always away from contact with 

 both basement membrane and distal 

 surface. 



These cells secrete the gas in much 

 the same way that it is done in Physalia except that the vacuoles of 

 gas must force their way to the surface and break out into the gas 

 chamber. 



An even more highly specialized form of gas cell is found in a third 

 form of siphonophore, Rhizophyza filiformis, which occurs in the Medi- 

 terranean Sea (Fig. 298). This large gas cell has a huge nucleus of 

 peculiar texture, which is shaped like a kidney. On the hollowed side 



FIG. 297. Two gas-secreting cells from 

 the gas epithelium of the siphonophore 

 medusa, Physalia. v., small vacuoles; 

 chr.v., the chromatic vacuoles: b., 

 bundles of mesodermal tissue belonging 

 to gas epithelium; b., smaller mesoder- 

 mal bundles belong to the corresponding 

 endodermal epithelium, the bases of 

 whose cells are indicated by lines. X 

 1000. 



