viii PRACTICAL DIRECTIONS 139 



f. Clearing and mounting. Place the section on a slide and warm 

 it gently on the water-bath until the paraffin melts, and then add a 

 large drop of turpentine in order to dissolve the paraffin. Then draw 

 off the turpentine with blotting-paper and replace it by a fresh drop, 

 repeating the process until all the paraffin is dissolved : put on a cover- 

 glass, and examine. 



If you wish to be sure that the parts of your sections are not displaced 

 in mounting, or to mount several sections on your slide, the latter should 

 first be smeared over with a very thin layer of a mixture of collodion 

 and oil of cloves, in equal parts: then place the sections on the slide, 

 warm, and immerse the whole slide in a small vessel of turpentine, 

 leaving it until all the paraffin is dissolved. 



In order to make a permanent preparation, remove the paraffin 

 with turpentine, as above, draw off the turpentine, place a drop of 

 Canada balsam on a cover-glass and very gently lower the cover-glass 

 on the object, spreading out the balsam in a thin, even layer. Before 

 long, the balsam will have set quite hard, and the sections may be pre- 

 served for an indefinite period ; the balsam will set more quickly if you 

 leave your preparations over the water-bath for a short time. 



Remember that object, razor, slide, and cover must be kept free 

 from water, the presence of which, from the stage of dehydration on- 

 wards, is fatal to success. 



Examination of Compound Tissues. 



Examine the following sections, prepared as described above, first 

 with the low, and then with the high power, noting the parts 

 enumerated in each case, as well as the structure of the nuclei (nuclear 

 membrane, chromatin, and micleoli]. 



1. Vertical section of skin (Fig. 38). 



a. Epiderm, stratified, divisible into outer (horny] and inner (Mal- 

 pighian) layers. 



b. Derm, connective-tissue fibres, blood-vessels and pigment-cells. 



c. Cutaneous glands with their ducts. The apertures of the ducts on 

 the surface you will probably have noticed already in your preparation 

 of the epithelial cells of the skin (p. 124). 



Sketch. 



2. Transverse section of intestine (Fig. 39). 



a. Mucous membrane : a superficial epithelial layer of columnar cells, 



