EPITHELIUM. j i 



solution of ammonium chromate for forty-eight hours. After washing, transfer it to picro- 

 carmine for twenty-four hours. Open the gut, and with a knife scrape off a little of the 

 epithelium and diffuse it in a drop of glycerine on a slide, cover and examine. 



EXAMINATION (H). Observe the tall, relatively large, columnar cells, with a fine striated 

 disc on their free ends, and a large red-stained nucleus. A careful examination reveals the 

 plexus of fibrils. Besides these cells, look for ' goblet ' or ' chalice ' cells, which are somewhat 

 cup-shaped, with a nucleus situated near the narrow, often tailed, end of the cell, and surrounded 

 by a small quantity of protoplasm, while the upper part of the cell appears empty. These 

 cells secrete mucin, so that they may be regarded as unicellular glands. The mucin seems to 

 be derived from the interfibrillar substance (PI. II., Fig. 7). 



3. CILIATED EPITHELIUM. 



A. LIVING CILIATED EPITHELIUM AND CILIARY MOTION. 



PREPARATION. Kill a frog by giving it a sharp blow on the head, but do not give 

 chloroform, for it paralyses ciliary motion. Scrape the mucous membrane of the roof of the 

 mouth or gullet, and diffuse the scraping by means of two fine needles in \ per cent, salt 

 solution. Place a hair in the fluid to prevent the cells being crushed, cover and examine. 



EXAMINATION (H). Observe the groups of short columnar or nearly spherical cells 

 with very granular contents, and having on one end vibratile cilia in active movement. This 

 movement causes currents in the surrounding fluid, which carry along with them any suspended 

 particles, such as granules or blood-corpuscles. Sometimes a detached cell may be observed 

 to spin round and round by the motion of its own cilia. The cilia themselves which are 

 fine homogeneous projections are best seen when their movements become languid. 



EFFECT OF REAGENTS. Irrigate with a drop of magenta solution, and note that the 

 colouring-matter does not stain the cells until the cilia have ceased to move. Thus each cell 

 while living can regulate its own nutrition. There is a marked difference, therefore, in respect 

 to colouring-matters between a living and a dead cell. 



B. CILIARY MOVEMENT. 



For the study of 'ciliary movement' the cilia of the common salt-water mussel are best 

 adapted. On opening the mussel with a knife, the yellowish-coloured gills are seen, and if a 

 small part be snipped off and placed in the salt water found in the inside of the shell, teased 

 with needles, covered and examined with a low power (L) the gills are seen as a series of 

 bars fringed with cilia, much larger than those of the frog's epithelium. (H). Observe the long, 

 tapering, clear, and structureless cilia bending most at their tips. Seal up this preparation by 

 running a ring of oil round the margin of the cover-glass, and set it aside for several hours till 

 the movement of the cilia becomes more languid (PI. II., Fig. 10). The oil prevents the access 

 of fresh oxygen, which is necessary for the ciliary motion ; and as the oxygen in the fluid 

 becomes used up, the ciliary action becomes slower. Place the slide on a hot stage (p. xxviii) 

 and gradually raise the temperature to not higher than 35 C. As the temperature rises, the 

 ciliary movement becomes more rapid, but if the temperature be too high, so as to coagu- 

 late the albumen, a permanent arrest of the movement results. An opposite effect is pro- 

 duced by the action of chloroform or ether. Place a fresh piece of the gill on a cover-glass 



c 2 



