HISTOLOGY: GAS-GLAND 391 



1940). In the present investigation gas-glands from seven specimens of Physalia were examined, 

 either in sections or in whole mount preparations. Some new observations have been made, but 

 interpretation has been difficult, and a further analysis is needed. The appearance of the cells in the 

 gas-gland varies markedly from one specimen to the next, and it is not clear to what extent this 

 variability is due to differing ages of the specimens, differing physiological states at the time 

 of fixation or to differing methods of fixation. A new investigation should take account of all these 

 factors. 



In a young specimen (^ = o-6cm.) the gas-gland achieves a thickness of 60 fi. The surrounding 

 saccus has a thickness of about 10 ju. The gas-gland consists of ectoderm, mesogloea and endoderm, 

 all of which are continuous with but clearly distinguishable from the the corresponding layers of the 

 surrounding tissue. Ectoderm and endoderm consist of tall columnar cells. They are separated by a 

 thick mesogloea ; elsewhere in the saccus the mesogloea is thin, and the cells flat. Muscle fibres are 

 present in both ecto- and endoderm, but are infrequent in the ectoderm. They run for the most part 

 circularly in the ectoderm and radially in the endoderm. Cytoplasmic processes cross the mesogloea 

 here, as elsewhere in the saccus. No nerves are present. 



The endoderm of the gas-gland consists of cells of one type, resembling the cells of the surrounding 

 regions in size, number of nucleoli and staining properties of nucleus and cytoplasm, but they are 

 unflattened and closely packed together forming a columnar epithelium. They each bear about 4-10 

 flagellae arising from basal bodies located in their distal tips. Around the edge of the gas-gland the 

 endoderm merges into the flattened cells characteristic of the remainder of the saccus. The endoderm 

 of the gas-gland, then, appears to be an unspecialized tissue, differing only from the general endoderm 

 of the float in being columnar, rather than squamous. There is no reason to suppose that it functions 

 actively in gas secretion. 



The ectoderm of the gas-gland shows a greater degree of specialization than the endoderm. This 

 is also the case in the other Siphonophora which have been examined. The cells of the gas-gland ectoderm 

 are sharply delimited from those of the surrounding regions. At least three types of cell have been 

 identified, each with cytological characteristics distinct from those of the remaining saccus ectoderm 

 cells. The cuticle (pneumatocyst) is absent over the gas-gland ectoderm, and the musculature 

 reduced. 



The three types of cell occurring in the gas-gland ectoderm are : (a) tall columnar cells forming the 

 bulk of the tissue ; (b) giant cells, scattered sparsely throughout the tissue, being particularly evident 

 in younger specimens; (c) a third type ('islet cells') occurring in clusters of three and upwards, 

 characterized by their denser cytoplasm and darkly staining, deep-lying nuclei. Of these three types, 

 only the first has hitherto been described in Physalia. Giant cells have been described in various 

 Physonectae and in other Cystonectae. Islets cells have no known counterpart in other Siphonophora. 



(a) The columnar cells 



Dahlgren and Kepner's description of gas-secreting cells in Physalia refers to cells of this type. 

 According to these authors a gaseous secretion develops in the region of a ' chromatic vacuole ' distad 

 of the nucleus, originating from granules which swell and become filled with gas. Gas-bubbles 

 rupture the cell-wall and break into the gas-chamber. 



A study of the Canary material has revealed the existence of chromatic vacuoles in only one speci- 

 men, a young one (g=o-8 cm.) fixed in Ca-formaldehyde. The gland was post-chromed for 36 hr., 

 sectioned in paraffin and stained in iron haematoxylin. The chromatic vacuoles are not as regular in 

 size and shape as those figured by Dahlgren and Kepner, but they are recognizably the same structures. 



