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 

 t ^ iat wn * cn 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 



FIG. 297. Two gas-secreting cells from , . ,. , .. 



the gas epithelium of the siphonophore single row, the undifferentiatcd cells 

 medusa, Physalia. v., small vacuoies; form a thick, many-layered mass in 



chr.v., the chromatic vacuoies: b., i i -i 11 t j i 



bundles of mesodermal tissue belonging wmch the g as cells are P laced Sparingly 



b., smaller mesoder- 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 vacuoies 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 



to gas epithelium; 



mal bundles belong to the corresponding 



endodermal epithelium, the bases of 



whose cells are indicated by lines. X 



1000. 



