GENERAL HISTOLOGY: CELLS AND TISSUES 27 



drawing already made. Use a small amount of the stain and wait patiently 

 for it to take effect. Much better results will be obtained than by piling on a 

 lot of stain in an attempt to hurry matters. To stain, place a drop of the stain 

 in contact with one edge of the cover glass, and draw it under by applying a 

 piece of filter paper to the opposite edge of the cover glass. (6) If what you 

 see in your preparation does not correspond with the description in the outline, 

 then you should promptly conclude that you are looking at the wrong thing and 

 should seek the assistance of the laboratory instructor. The descriptions in 

 the outline have been made as accurate as possible. (7) Draw only a few cells, 

 making your drawings large and detailed, putting in every structure that you 

 can see. 



i. Epithelial tissues. This kind of tissue covers or lines all the free surfaces 

 of the body, and is further distinguished by the relatively unspecialized char- 

 acter of its cells, which are similar in structure and appearance to the "typical" 

 cells already described, and by the almost complete absence of intercellular 

 substance. The cells are united into continuous sheets by a cement substance, 

 which is difficult to demonstrate. There are several kinds of epithelia (Holmes, 

 pp. 121-22). 



a) Squamous epithelium: Obtain a small piece of shed epidermis (outer layer 

 of the skin) of the frog, spread it out on a slide in a drop of water, cover with a 

 cover glass, and examine with the low power. Turn down the light. Note the 

 polygonal cells of which it is composed, giving a characteristic mosaic appear- 

 ance; these cells are found to be very thin and flat when viewed from the side. 

 Study a cell with the high power, note the nucleus and (in some cases) the pig- 

 ment granules in the cytoplasm. If the nucleus is not clearly visible, stain with 

 a drop of aceto-carmine. (See general directions for method of staining.) By 

 changing the focus of the microscope determine whether the epidermis is one 

 or more layers of cells thick. Draw, showing a few of the cells. 



b) Columnar epithelium: In contrast to the preceding, this type of epithelium 

 is characterized by the tall and slender shape of its cells. Obtain a small piece 

 of the inner lining of the small intestine which has been macerated for twenty- 

 four hours in 5 per cent chloral hydrate, add a few drops of salt solution, tear it 

 into the smallest possible bits with a pair of teasing needles, cover and examine 

 with the high power. Look for slender cells, slightly broader at one end and 

 narrower or irregularly branched at the other. The oval nucleus occupies an 

 enlargement which is usually nearer the narrow end. In some of the cells the 

 broader end will be found to contain a cup-shaped cavity which in life is filled 

 with mucus. Such cells are called goblet cells. Draw a few of the cells. If the 

 nucleus is not visible, stain with a little aceto-carmine. In their natural position 

 these cells form a single layer lining the cavity of the intestine, their long axes 

 parallel to each other, and their broad ends facing the cavity. 



