326 



HISTOLOGY 



BREMER, J. L. "On the Lung of the Opossum," Am. Journ. of Anat., 1904, Vol. Ill, p. 67. 



MILLER, WM. S. "The Lung of the Salamander, Necturus," Bull, of the Univ. of Wis- 

 consin, Nr. 33. 



PLATE, LUD. H. "Studien iiber Opisthopneumone Lungenschnecken," Zool. Jahrb. 

 Abt. fur Anat., Band IV. 



HOLMGREN, E. " Uber das respiratorische Epithel der Trachcen bei Raupen," Festsk. 

 Lilljeborg., Upsala, 1896, pp. 79-96. 



FIG. 291. Transverse section of an embryonic respira- 

 tory filament of Acanthias. X 400. 



WATER-BREATHING RESPIRATORY TISSUES, GILLS 



The water-breathing forms of respiratory tissues are the most primi- 

 tive ; at the same time their distribution is most diverse and their varia- 

 tion is greatest. 



As an example of a simple water-breathing, respiratory organ we 

 shall take the primary gill filaments of the embryo of Acanthias vulgaris 



(Fig. 291). In the em- 

 bryo of most selachians 

 the animal secures its 

 oxygen and carbon dioxide 

 exchange, with the embry- 

 onic fluids in which it lies, 

 by means of a series of 

 long filaments that grow 

 out from the sides of the 

 neck in the gill region. 

 Each filament consists of a long, single capillary loop embedded in a 

 very small amount of connective tissue, and surrounded by an evaginated 

 layer of the body epithelium. The blood passes down one side of the 

 filament and returns on the other. The afferent and efferent capilla- 

 ries that are seen in the section of the filament are lined with a single 

 layer of endothelial cells and contain fully developed, red, nucleated 

 corpuscles. 



In the somewhat oval section of such a filament we find the respiratory 

 cells to be a single layer of cells, a little too flat to be called cubical, and 

 differing but little from the cells on the surface of the body from which 

 they were derived, except that these latter are already stratified in a 

 four-centimeter embryo into two layers or more. 



The small amount of connective tissue that is seen, forms, for the 

 most part, a septum separating the two vessels from one another. A 

 few of these cells are to be found between the capillary and the respiratory 

 epithelium but they are very much flattened. Undoubtedly parts of 

 the cytoplasm of these cells separate the vessels from the epithelium at 

 every point. The triple wall of epithelium, connective tissue, and en- 

 dothelium is a fairly efficient organ for the transmission of gases when 



