GILLS 



325 



disk-shaped nuclei (Fig. 290). They bear on their inner surfaces a layer 



of cuticle which they have elaborated. As we saw was the case in the 



water-breathing crustacean gill, this cu- 



ticle has undergone no further modifi- 



cation than to become as thin as possible. 



In the gill of the Crustacea the pressure 



of the blood was positive and inflated 



the evaginated structure. Here the 



pressure of the ccelomic fluids is nega- 



tive, and tends to collapse the tube. 



In these invaginated tracheal tubes, 



therefore, the cuticle has been modified 



in an important manner. It has been 



thrown into thick, circular ridges, the 



tanidia. These serve to keep the deli- 



cate tubes open, and at the same time 



do not make the walls unnecessarily 



thick and heavy. Similar but not 



homologous formations are met with 



in the tube that leads to the large lung FIG. 290. Part of an oblique section of 



sacs of the mammals and other verte- 



bratCS. In this case there is also a 



negative pressure during expiration and 



cartilaginous rings are developed to 



keep it from collapsing. The conception of the trachea as invaginations 



of the outer surface is abundantly borne out by the embryological work 



on this structure. They can be seen in all stages of invagination. They 



do not, at first, contain air, this appearing at an early stage. 



Technic. One must be somewhat more careful in fixing these 

 tissues than in dealing with the water-breathing tissues. When once 

 fixed and hardened the treatment is practically the same. When pos- 

 sible, the fixation should be done under a gentle pressure. The harden- 

 ing as well as the fixation should be of some duration. Often it is of 

 great advantage to fix by inflating the organs with the fumes of osmic 

 acid and, after this has had plenty of time to act (long enough to "osma- 

 tise" the respiratory cells), the fixation can be finished by the 'use of any 

 other fixative. Silver nitrate used on the fresh tissues may be made to 

 show the cell outlines very beautifully. 



tube; /., lateral view of the same; /., 



^ S 6o r tenidia of the cuticular tube ' 



LITERATURE 



MILLER, W. S. " Das Lungenlappchen seine Blut- und Lymph-gefasse," Arch. f. Anat., 



1900, S. 197. 

 OPPEL, A. "Atmungs-Apparat," Erg. d. Anat. und Entwickl., 1902, Band XII, S. 134. 



