EPITHELIUM. 9 



EXAMINATION (H). The squames are flat, horny, and transparent, and usually no 

 nucleus is visible. 



EFFECT OF REAGENTS. Irrigate with a five per cent, dilution of liquor potassae ; the cells 

 swell up and become globular. Neither this nor the previous preparation need be preserved. 



INTRA-CELLULAR PLEXUS OF FIBRILS. 



It has recently been shown that the nucleus of cells contains a delicate plexus of fine 

 fibrils an intra-nnclear plexus which is continuous with a similar plexus in the perinuclear 

 protoplasm the intra-cellular plexus. It is this plexus, when the ends of its fibres are directed 

 towards the observer, that gives the cells the appearance usually described as ' granular.' 



PREPARATION. Keep a newt in a small quantity of water for several days, but do not 

 change the water. At the end of this time the superficial layer of cutaneous epithelium will 

 slough off in the form of a fine film. Harden this film for twenty-four hours in a saturated 

 solution of picric acid, or in absolute alcohol. Preserve both in alcohol. When required for 

 use, snip off a small piece and stain it, if hardened in picric acid, with picrocarmine for half 

 an hour, while the piece hardened in alcohol may also be stained with logwood. Mount in 

 Farrant's solution. 



EXAMINATION (H). Observe the cells united by their margins. Within each polygonal 

 area notice the oval red-stained nucleus : study it carefully, and a delicate plexus of fibrils 

 the intra-nuclear plexus will be seen (PI. II., Fig. 2). 



Both in mucous membranes and in the skin, epithelium occurs in several layers, constituting 

 stratified epithelium. In order to see the deeper layers, and to study the relation of one layer 

 to another, sections must be made. A section of the conjunctival epithelium of the cornea is 

 convenient for this purpose. 



Vertical section of the conjunctiva and cornea for stratified epithelium. 



PREPARATION. Remove the cornea from the eye of a cat just killed and place it in 

 40 c.c. of chromic acid and spirit mixture (p. xxxi) consisting of two parts of a sixth per cent, 

 solution of chromic acid, and one part of methylated spirit. Change the fluid at the end of 

 the first, fourth, and seventh days ; on the tenth day it will be sufficiently hardened. For 

 preservation transfer it to methylated or rectified spirit, after washing away all the surplus 

 chromic acid. When sections are wanted, put the cornea in a large glass of water for 

 twenty-four hours to remove the spirit. Transfer it to a solution of gum (p. xxxviii) for twenty- 

 four hours and then cut vertical sections in a freezing microtome (p. xxxviii). After the sections 

 are made, they are placed in a large quantity of water for twenty-four hours, to get rid of 

 the gum, when they may be put into the preservative glycerine fluid (p. xl) till they are 

 required for mounting. 



EXAMINATION (H). Float a section on to a slide (p. xxvi), add a drop of solution of 

 picrocarmine (p. xliii), and allow it to remain on the section for fifteen minutes, or until the 

 section is sufficiently stained. Soak up the surplus staining-fluid with blotting-paper, and add 

 a drop of Farrant's solution. Cover and examine. The fibrous cornea is stained red, but 

 neglecting this, observe the layers of epithelial cells, stained yellow and their nuclei red, 

 covering its anterior surface. The deepest layer consists of cells columnar in shape ; those 

 on the surface appear as little more than lines or very elongated spindles, whilst between 

 these extremes there are several layers of cells which show intermediate forms. The cells 

 are developed from below from the lowest layer and are gradually pushed upwards, and 

 hence the change of shape. {Indicate these characters in PI. II., Fig. 5). 



C 



