24 THE ESSENTIALS OF HISTOLOGY. 



LESSON VI. 



EPITHELIUM. 



1. MOUNT a drop of saliva and examine first with a low, afterwards with a 

 high power. Observe the nucleated epithelium-cells, some single, and others 

 still adhering together by overlapping edges. Measure three or four, and also 

 their nuclei. Sketch one or two on the flat and one edgeways. Notice the 

 salivary corpuscles, which are like white blood-corpuscles swollen out by 

 imbibition of water. 



2. Put a small shred of human epidermis into a drop of strong caustic potash 

 solution for five minutes. Then break it up in water with needles, cover 

 and examine. Observe the now isolated swollen cells. M easure some. 



3. Study the arrangement of the cells in a section through some stratified 

 epithelium, such as that of the mouth, skin, or cornea. 1 Notice the changes 

 in shape of the cells as they are traced towards the free surface. Measure 

 the thickness of the epithelium. Count the number of layers of cells. 



4. Study the minute structure of epithelium-cells and their nuclei, both 

 at rest and dividing, in sections of the skin of the newt's tail or in shreds of 

 epidermis of the salamander-tadpole. The preparation may, for this purpose, 

 be stained either with hsematoxylin or with some aniline dye such as 

 safranin. 2 



Sketch an epithelium -cell with resting nucleus, and others with nuclei in 

 different phases of karyokinesis. 



An epithelium is a tissue composed entirely of cells separated by a 

 very small amount of intercellular substance (cement-substance), and 

 generally arranged so as to form a membrane covering either an 

 external or an internal free surface. 



The structure of epithelium-cells, and the changes which they 

 undergo in cell-division, are best seen in the epidermis of the newt 

 or of the salamander-tadpole; in the latter especially the cells and 

 nuclei are much larger than in mammals. 



Structure of the cells. Each epithelium-cell consists of protoplasm 

 containing a nucleus. The protoplasm may either look granular, or it 

 may have a reticulated appearance. In some kinds of epithelium it is 

 striated. The nucleus is a round or oval vesicle lying in the proto- 

 plasm. Usually there is only one, but there may be two or more. 



1 The methods of preparing sections are given in the Appendix. 



2 The methods which serve the purpose of exhibiting the division of nuclei are 

 given in the Appendix. 



