19 



LESSON VI. 



EPITHELIUM. 



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

 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 one or two minutes. Then break it up in water with 

 needles, cover and examine. Observe the now isolated swollen cells. 

 Measure some. 



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

 epithelium, such as that of the mouth, skin, or cornea. The section may be 

 prepared beforehand by the demonstrator ; it should be mounted in Canada 

 balsam. 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 the tail of the salamander-tadpole. 2 This preparation 

 may also advantageously be prepared beforehand by the demonstrator. 



[The preparation is made as follows : The tail is placed in chromic acid 

 solution (Ol per cent.) for three days, then thoroughly washed for some hours 

 in water to remove the excess of the acid, then placed in dilute haematoxylin 

 solution for twenty-four hours, or in 1 p. c. safranin solution for a similar 

 time. After having been again rinsed in water it is rapidly dehydrated 

 in absolute alcohol, then transferred to spirits of turpentine, and finally 

 mounted in Canada balsam.] 



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

 different phases of karyomitosis. 



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 ex- 

 ternal 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 tail of 

 the salamander-tadpole, in which the cells and nuclei are much larger 

 than in mammals. 



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



2 If these tadpoles are not obtainable, the structure may be studied in the 

 epidermis of the newt, prepared as recommended in Lesson VII. 1 for the study 

 of columnar epithelium, and also in sections stained with haematoxylin. 



c2 



